PDF file signature
Magic number at the start of the file · %PDF-
A PDF begins with %PDF- followed by its version, for example %PDF-1.7. Modern Adobe Illustrator (.ai) files are PDF-compatible and start with the same bytes, so they are detected as PDF.
What this magic number means
A magic number is a short, fixed run of bytes at a known position that tells a program what a file really is, no matter what the filename says. If a file starts with the bytes 25 50 44 46 2D (the text %PDF-), it is a PDF file.
End-of-file marker
This format is identified by a trailer, not a header. The bytes 25 25 45 4F 46 appear near the end of the file (%%EOF marks the end of a PDF).
Formats that use the 25 50 44 46 signature
These file types in our database carry this signature:
How to check a file's signature
You can read the first bytes of any file yourself. The magic bytes are shown in hex, the same way this page lists them.
Linux & macOS
- xxd -l 16 example.pdf
- hexdump -C -n 16 example.pdf
- file example.pdf
Windows (PowerShell)
- Format-Hex -Path example.pdf -Count 16
Python
- open("example.pdf","rb").read(5).hex()
In your browser
- Drop the file into the WhatFileType identifier, which reads the signature without uploading it.
Frequently asked questions
What is the PDF file signature?
PDF files start with the hex bytes 25 50 44 46 2D (%PDF- in ASCII). This magic number identifies the format regardless of the file's name or extension.
How do I check a file's magic number?
Open the file in a hex editor, or run a command such as xxd -l 16 example.pdf on Linux or macOS, or Format-Hex -Path example.pdf -Count 16 in Windows PowerShell, and read the first bytes.
Can a file fake the PDF signature?
Renaming a file does not change its bytes, so the extension can lie but the signature usually cannot. A genuine PDF file has these exact bytes; a file with the wrong bytes is not really PDF, whatever its name says.