Of course, viewing is one thing; editing is another process. If you want to write something in an existing PDF file, and then save your changes, you have software such as, PDF Architect, iSkysoft ...

How to Write Notes on PDF Files. The Sticky Note tool in Adobe Acrobat and Adobe Reader allows for commenting on your business PDF files. The tool can display notes in multiple colors and styles ...

To write on a PDF on Android, open the PDF file in the Adobe Acrobat Reader: Edit PDF app, and tap on the Text icon. Select where you want to input new text and type away.