Although PDF files are often encrypted to dissuade anyone from editing their contents, several online tools can be used to undo its PDF formatting. For instance, one site has a free tool that undoes PDF formatting and allows anyone to type on a PDF.
The typewriter tool in Adobe Acrobat also has a functionality to annotate PDF documents, though the usage rights must allow for this to be enabled. This can be enabled by going to the "advanced" tab and enabling usage rights. Certain text documents can also be converted into Microsoft Word or other text file formats, using a feature in Adobe Acrobat Pro.