Let me put forward a problem. You've missed a lecture but you've got the lecture slides on a PDF document. You want to write your notes on them to help you better understand them at a later date... and you can't. Lecturers often seem to put things in PDF form, and whilst they can be highly useful, they're fairly limited to what you can do with them. After a bit of research (Googling, essentially, why do we even need the "r" word anymore?) I've found some handy bits and pieces no student should live without.
PDF Hammer allows you to upload a PDF document and apply basic editing to it. You won't be able to edit text or graphics, but it'll allow you to modify the pages as whole entities. You'll be able to move pages around, delete pages that aren't necessary or useful, and even merge documents together into one big PDF. At the end, it'll allow you to export it to your computer with no trouble, problem or added anxiety. Simple and genius.
If something you prefer is offline editing, PDFTools does very much the same as PDF Hammer except offline, with an added XML to PDF function which the former doesn't offer. Still, why bother installing something when it works just as well as a web based application? Beats me.
Nitro PDF Professional does cost money, I'll admit, but it's brilliant. You'll often get PDF's through from lecturers and professors, and most of it is waffle and not necessary. Although the converting from PDF to Word isn't great, the interface which is styled in a Ribbon "Office 2007" style, makes it easy to edit your files using the WYSIWYG editor. It'll cost $99 for the full version, but they offer a trial version which lets you play with all the features available, but this is by far the best PDF editor on the market.
For those who use Linux, PDFedit is a SourceForge project in constant development, which enables you to many of the features that Nitro PDF would offer, but it's free and obviously, works on Linux.
Finally, considering most computers at university, although still running Windows XP, are adopting Office 2007. If you've bought a laptop over the last year, the chances are Office 2007 will be bundled with it as well. Downloading and installing the Microsoft Save as PDF or XPS tool which plugs right into Office, allows you to save any document you can open into a PDF format for easy distribution. You will need to be running a genuine copy of Office for it to install though.