TL;DR
Someone sends you a file. Double-clicking does nothing useful, or your OS offers to search the App Store. The extension means nothing to you.
Before you paste it into the first "open any file online" site Google shows you — which is the reflex, and a bad one — do two things:
- Identify what it actually is. Most unknown files are plain text, structured text, an image, or an archive wearing an unfamiliar extension. A file's extension is a label, not a fact.
- Decide whether opening it is safe. Some file types execute code. Identification tells you which category you're in.
Once you know it's text, markup, tabular data, or an image, converting it to PDF is local and instant with Convert: Anything to PDF — JPG, PNG, WebP, SVG, GIF, BMP, TXT, HTML, Markdown, JSON, XML, and CSV all convert in the browser with no upload.
If it turns out to be a proprietary binary — .docx, .xlsx, a CAD file, a database — no browser extension will help, and this post will tell you that instead of wasting your time.
Step 1: The extension is a hint, not an answer
File extensions are just characters after a dot. Anything can be renamed to anything. What determines a file's actual type is its magic number — the first few bytes, which follow a convention per format.
You can check this without installing anything.
macOS / Linux:
file mystery-document.2026
You'll get something like ASCII text, JSON data, PNG image data, 1200 x 800, or Zip archive data.
Windows PowerShell — read the first bytes as hex:
Get-Content -Path .\mystery.2026 -Encoding Byte -TotalCount 8 | ForEach-Object { "{0:X2}" -f $_ }
Common signatures:
| First bytes (hex) | Actual format |
|---|---|
25 50 44 46 | PDF (%PDF) |
FF D8 FF | JPEG |
89 50 4E 47 | PNG |
47 49 46 38 | GIF |
52 49 46 46 … 57 45 42 50 | WebP |
42 4D | BMP |
50 4B 03 04 | ZIP container — or .docx, .xlsx, .pptx, .epub |
3C 3F 78 6D 6C | XML (<?xml) |
7B or 5B | Likely JSON ({ or [) |
| Printable ASCII | Plain text of some kind |
That 50 4B 03 04 row catches people out constantly. Modern Office documents are ZIP archives containing XML. If your mystery file is a ZIP, it might be an archive — or it might be a Word document someone renamed.
No terminal available? Rename a copy to .txt and open it in a text editor. If you see readable content, it's text. If you see a wall of replacement characters with occasional recognizable words, it's binary. Do this on a copy.
Step 2: Decide whether to open it at all
This is the part the "open any file online" sites skip entirely, and it matters more than the conversion.
Generally safe to open: plain text, JSON, XML, CSV, Markdown, and standard image formats. These are parsed as data. Image parsers have had vulnerabilities historically, but on a patched browser the risk is low.
Safe to open in a browser with care: HTML. It can contain scripts, but the browser sandbox is designed for exactly that. Don't open a local HTML file from an untrusted sender expecting nothing to happen.
Do not open casually: anything executable or macro-capable. .exe, .msi, .dmg, .pkg, .app, .scr, .bat, .ps1, .sh, .jar, .docm, .xlsm, and — worth calling out this month — notebook files. Reporting from the week of July 21–27, 2026 covered GitLab remote-code-execution flaws exploitable via malicious Jupyter Notebooks. A .ipynb is JSON, which sounds harmless, but it's JSON that a notebook environment will happily execute.
That's the general lesson: "it's just text" describes the format, not the risk. The risk comes from what will interpret it. JSON opened in a text editor is inert. The same JSON opened in an execution environment is a program.
If the file came from someone you don't know, or arrived unexpectedly from someone you do, stop here. Verify through a channel you chose — a phone number you looked up, not one in the message. Unknown attachments are the oldest delivery mechanism there is and they still work.
Step 3: Convert, by category
It's an image
JPG, PNG, WebP, GIF, BMP, and SVG all convert directly. Select the file in Convert: Anything to PDF and export.
If it's SVG, it converts as vector rather than being rasterized, so it stays crisp at any zoom — relevant for logos and diagrams (more here).
If it's a scan of a document, be clear about what you're getting: an image inside a PDF. The text in it stays unsearchable, because there's no OCR in the extension. For a searchable result you need Acrobat, macOS's built-in text recognition, or an OCR service.
It's plain text or Markdown
TXT converts directly. Markdown renders as formatted output — headings, lists, code blocks, emphasis — rather than raw syntax. Useful for notes, READMEs, and exported documents (Markdown to PDF).
It's JSON or XML
Both convert with structure and indentation preserved, which is the point — a 4,000-line API response as an unwrapped single line is unreadable, and as a formatted PDF it's reviewable and annotatable. This is a common need for audit logs and API payloads (JSON and XML to PDF).
If your JSON is minified, run it through a formatter first — the extension preserves the structure it's given rather than reformatting for you.
It's CSV or tab-separated
CSV renders as a real table with header styling and aligned columns. For wide data, set landscape and A3 or Ledger before converting, or the right-hand columns get clipped (wide table fix).
Tab-separated data isn't a distinct supported type — convert it to comma-separated first, or open it in a spreadsheet and export as CSV.
It's HTML
A local HTML file converts directly. Two caveats: if it references external stylesheets or images by relative path and you've moved it away from its folder, it'll render unstyled. And if it was saved from a website, you may be better off revisiting the page and using Convert: Web to PDF on the live version instead.
It's a ZIP container
Extract it and identify what's inside. If it's .docx, .xlsx, or .pptx, you need real Office software — see below. If it's an archive of images or text files, convert the contents and merge them into a single PDF.
It's a proprietary binary
.docx, .xlsx, .pptx, .psd, .ai, .dwg, .sketch, database files, and similar all need software that understands the format. A browser extension can't parse them, and we're not going to claim otherwise.
Your options: the originating application, a free equivalent (LibreOffice reads most Office formats), or the platform's own export. For Excel data specifically, export to CSV and convert that — the documented workflow, not a workaround.
Why local conversion matters more for unknown files
The instinct with a file you can't identify is to upload it somewhere and let a service figure it out. Consider what you're doing: taking a file of unknown provenance and unknown contents, and handing it to a third party.
Two problems. If the file is sensitive, you've just disclosed it to a service whose retention policy you haven't read. Unknown files from work contexts are frequently exports, logs, or reports containing things you wouldn't email externally.
If the file is hostile, you've done nothing to protect yourself — you've simply moved where it gets parsed, and you're still going to download and open whatever comes back.
Local conversion doesn't magically make a hostile file safe. But it keeps a sensitive file private, and it removes an entire third party from a situation that already has too many unknowns. You can verify the "local" claim in ninety seconds: DevTools → Network, clear the log, run a conversion, watch nothing leave.
Quick reference
| Symptom | Likely type | Convert with extension? |
|---|---|---|
| Opens readably in a text editor | TXT / Markdown / JSON / XML / CSV | Yes |
Starts with { or [ | JSON | Yes |
Starts with <?xml | XML | Yes |
Starts with <html or <!DOCTYPE | HTML | Yes |
| Comma-separated lines, consistent columns | CSV | Yes |
file says "image data" | Image | Yes |
file says "PDF document" | Already a PDF | Not needed |
file says "Zip archive" | Archive or Office file | Extract first |
| Gibberish in a text editor, no known signature | Proprietary binary | No |
Extension is .exe, .msi, .dmg, .bat, .ps1, .jar | Executable | Don't open |
Extension is .ipynb, .docm, .xlsm | Executable content | Don't open casually |
Frequently asked questions
How do I find out what an unknown file actually is?
Check its magic number — the first few bytes, which identify the real format regardless of the extension. On macOS or Linux run file yourfile. On Windows, read the first bytes as hex in PowerShell. Or, on a copy, rename it to .txt and open it in a text editor: readable content means it's text.
Can a file's extension lie about what it is?
Yes. Extensions are just characters after a dot and can be changed to anything. That's why identification by magic number matters — particularly for .docx, .xlsx, and .pptx, which are ZIP archives internally and share a signature with ordinary ZIP files.
Is it safe to open a file with an extension I don't recognize?
Depends entirely on the type. Text, JSON, XML, CSV, and standard images are parsed as data and are generally safe on a patched system. Executables and macro-capable formats are not. Notebook files deserve particular caution — a .ipynb is JSON, but a notebook environment will execute it, and GitLab RCE flaws exploitable via malicious notebooks were reported in late July 2026.
Can Convert: Anything to PDF open any file type?
No, and the name is about breadth of common formats rather than literally everything. It handles JPEG, PNG, WebP, SVG, GIF, BMP, TXT, HTML, Markdown, JSON, XML, and CSV, plus the active web page. Proprietary binaries like .docx, .xlsx, .psd, and CAD files need their own software.
What if my file turns out to be a Word or Excel document?
Use software that understands the format: Word or Excel, LibreOffice (free, reads both), or Google Docs and Sheets. For Excel data you want as a PDF table, export to CSV and convert the CSV — that renders as a formatted table with proper column alignment.
Should I upload the file to an online identifier instead?
Not as a first step. You'd be handing a file of unknown contents to a third party — a disclosure risk if it's sensitive, and no protection at all if it's hostile. Identify it locally first with file or a text editor; that answers the question in seconds without the file leaving your machine.
My file is a scan. Can I make the text searchable?
Not with this extension — it has no OCR. The scan converts into a PDF containing the image, and its text stays unselectable. Use Acrobat, macOS's built-in Live Text, or a dedicated OCR service if you need searchable output.
Does converting a file upload it anywhere?
No. Conversion runs entirely in your browser with no server round-trip and no account. Verify it yourself: open DevTools, go to the Network tab, clear the log, and run a conversion — nothing leaves.
Bottom line
An unfamiliar extension is a question about the file's real type, and it's answerable locally in about ten seconds. Most of the time the answer is "text, structured text, or an image," and those convert to PDF in the browser with nothing uploaded.
The part worth being disciplined about is the security check before the conversion. Identify first, decide whether to open second, convert third. And if the file arrived unexpectedly, verify with the sender through a channel you chose before doing any of it.
Convert: Anything to PDF handles the twelve formats above plus merging, free, with no account and no watermark. When the file turns out to need real Office software or OCR, we'd rather point you there than pretend.
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