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 ...

The file will open in PDF format automatically if you selected the Open File After Publishing option. Otherwise you can open your proposal by navigating to and double-clicking the new PDF file.

Luckily, Adobe, the creator of the PDF, offers a super-simple and free online tool that will help you combine your files in a matter of minutes.