Encrypting PDF files is crucial for protecting sensitive information, and QPDF is a powerful command-line tool that makes this process simple. With just one command, you can secure your PDF files with password protection.
Why Use QPDF?
QPDF is a free and open-source tool designed for manipulating PDF files. It allows encryption, decryption, compression, and other modifications. It’s lightweight, easy to install, and supports strong encryption methods.
To use QPDF, you need to download it from its official repository: https://github.com/qpdf/qpdf
Once downloaded, extract the files and ensure the qpdf.exe is accessible from the command line. You may add the QPDF directory to your system’s PATH variable to use it from any location.
Encrypting a PDF with QPDF
QPDF enables encryption using a simple command format:
qpdf in-file out-file --encrypt user-password owner-password key-length --
- in-file: The original PDF file
- out-file: The new encrypted PDF file
- user-password: Password required to open the file
- owner-password: Password for full control (printing, copying, modifying)
- key-length: Encryption strength (40, 128, or 256 bits)
- —: Required to terminate the encryption parameters
The key length of 40 is considered weak. If you want to use 128 bits key then you should use AES encryption which is used automatically for the 256 bits key length.
Example Command
To encrypt sample.pdf with a user password of k7p9m2n4 and an owner password of j5w2r8m4 using 256-bit encryption, run:
qpdf sample.pdf encrypted.pdf --encrypt k7p9m2n4 j5w2r8m4 256 --
Here the user password is the password required to open the document and the owner password grants full control, allowing editing, printing, and removing encryption. You should use very strong passwords. You can generate very strong passwords using Norton Password Generator.
Verifying Encryption
After running the command, try opening encrypted.pdf. It should prompt for a password. If you entered the correct user password, you can view the file, but restrictions will apply unless the owner password is used. All the popular PDF viewer applications like Sumatra PDF, Foxit PDF Reader, Adobe Acrobat Reader etc., support this encryption.
Conclusion
QPDF is a simple yet powerful tool to encrypt PDF files with just one command. By using strong encryption (256-bit) and unique passwords, you can effectively protect sensitive documents on Windows.