If you have an application like Adobe Acrobat, then yes, you can edit a PDF document - as long as the PDF document is not protected and prevents such editing.
However, keep in mind that Acrobat is not a word processor, and things that are really easy to do in e.g. MS Word may be impossible in Acrobat. Oftentimes it's better and easier to convert the PDF file to e.g. a Word document (Acrobat can do that as well), and then edit the Word document and convert to PDF when done.
What types of edits are you trying to make?
Karl Heinz Kremer
PDF Acrobatics Without a Net
PDF Software Development, Training and More...
http://www.khkonsulting.com