.pdf — Portable Document Format
Document application/pdf
What a .pdf file is
A fixed-layout document format that renders identically everywhere. It can embed fonts, vector graphics, raster images, form fields and digital signatures.
How to open it
Any browser, plus Preview, Acrobat and hundreds of other readers.
Worth knowing
A PDF may contain selectable text or just a scanned image. If you cannot select the text, it needs OCR before it becomes searchable.
How to identify a .pdf file
A file's extension is only its name — renaming a.png to a.pdf changes nothing inside. What software actually reads is the magic number, the first few bytes of the file.
| Signature | 25 50 44 46 2D |
| MIME type | application/pdf |
| Category | Document |
ASCII "%PDF-" followed by the version.
# what is this file really?
file mystery.pdf
# look at the first bytes yourself
xxd mystery.pdf | head -2
head -c 16 mystery.pdf | xxd
# Windows PowerShell
Get-Content mystery.pdf -AsByteStream -TotalCount 16 | Format-HexFrequently asked questions
What is a .pdf file?
A fixed-layout document format that renders identically everywhere. It can embed fonts, vector graphics, raster images, form fields and digital signatures.
How do I open a .pdf file?
Any browser, plus Preview, Acrobat and hundreds of other readers.
Is .pdf safe to open?
Generally yes, though both formats can carry active content — macros in Office documents, JavaScript in PDFs. Modern readers disable these by default; leave it that way for files from strangers.
Can I change a file's extension to convert it?
No. Renaming changes the label, not the bytes. The one place it appears to work is between formats that genuinely share a container — renaming .docx to .zip works because a DOCX is a ZIP. Everything else needs a real converter.