Install and use Adobe Acrobat. Don't use Acrobat Reader.
Hi Kathryn -
You should be able to edit a document, unless that document is secured with a password to prevent editing changes. If you try with a couple of PDF files, and aren't able to use any editing tools, then choose Help > Repair Acrobat Installation (on Windows) to allow the program to go through its installed components and correct any programming errors that may have occurred.
Which version of Acrobat Pro are you using? You'll find the Edit tools in the right hand pane, and when active, you'll have lots of choices for editing content on the page.