.PDF file signature
A PDF document file begins with the bytes 25 50 44 46 2D (%PDF- in ASCII) at the start of the file. The signature is the same whatever the file is named, which is why a renamed or extensionless PDF file is still recognisable.
Signature
| Offset | Hex | ASCII |
|---|---|---|
| 0 | 25 50 44 46 2DFollowed by the version, e.g. %PDF-1.7. The trailer ends with %%EOF. | %PDF- |
MIME type: application/pdf · usual extensions: .pdf
How to check a file's signature
Without installing anything: drop the file on our detector. It reads the bytes in your browser and shows the hex dump alongside the verdict; nothing is uploaded.
macOS and Linux (Terminal):
xxd -l 32 file.pdf
file file.pdfWindows (PowerShell):
Format-Hex -Path file.pdf -Count 32If the bytes do not match
A file named .pdf whose first bytes are something else is not a PDF file. The common explanations are a wrong or deliberately misleading extension, a download that saved an HTML error page instead of the file, or a container such as ZIP wrapping the real content. The detector reports what the bytes say; the PDF guide explains what a genuine file should contain.