You haven't said which operating system you're using, but if you're using a Mac, you can create and edit .pdf files easily, and your wish to provide information in certain sections is a piece of cake with several cherries on top. These edits are readable on computers on all OS(es).
If you use
any version of Windows, however, you can create and edit .pdf files only when you have purchased Adobe's
Acrobat. Unfortunately, this software does cost a pretty penny, but these files are readable on other computers.
This site gives information on circumventing this hurdle -
http://www.labnol.org/software/edit-pdf-files/10870/. I don't know how solid this information is, but it's worth a try.
If you cannot edit these .pdf files on your computer, the only other solution is to print them out, fill in the sections as necessary, scan them into your computer and e-mail them as attachments, or send them through the post.