PDF files have the option to let you share and comment on documents in a wide range of applications. Adobe Acrobat Reader and other free PDF programs offer a subset of the capabilities of the paid ...

Open the PDF document that you want to stamp in Adobe Acrobat. Click the "Comment" tab on the right side of the menu bar at the top of the screen to open the Comments panel.

OCRmyPDF is a free command line tool for adding searchable and copyable text to any PDF file.