PDFEncrypt is a free, open-source native Windows app that allows you to password protect (encrypt) PDF files for free, without purchasing expensive software.
Download Installer without .Net Framework
This app uses the iTextPDF open source library for encryption operations, and is released under the AGPL license. Complete source code is available on GitHub, and you can download the installer for Windows 7, 8, 10, and 11 below.
PDFEncrypt is popular worldwide!