When someone shares a document with you that's in PDF format but you need to edit it, try the free online PDFConverter. It'll convert a PDF to PowerPoint (PPT), Word (DOC) or Excel (XLS), and you don't need to download any software to use it.
Just select the type of conversion, upload your file, enter your email address, and receive an email with a link to your converted file.
Many websites provide access to forms in PDF format. Unfortunately, not all PDF forms are coded to allow the user to fill them in online. You have to print out the form and then fill in the blanks by hand (blecch) or with a typewriter (scarce as hen's teeth.)