PDF, Portable Document Format, files are ubiquitous. They are everywhere. They are a great way to share and publish files since they can be locked, watermarked, and viewed on pretty much any device or OS. Most office suites allow you to save files as PDF formats and there are some other great tools for creating them.
Here are some great resources for working with PDF files and an infographic about PDF files and the format.
Lots of PDF resources - print, markup, convert and more
Crocodoc - markup PDF files for free
Fill Any PDF form - fill out, sign and send forms
I Love PDF - merge or split PDF files
Adobe Digital School Collection - supporting creativity and digital literacy - includes Adobe Acrobat for creating PDF files