You are able to type in some PDF documents, but the author has to enable this when the create the PDF. With Adobe Reader (or Foxit, etc...) you can't edit anything, but you are able to type in to question boxes where appropriate.
Adobe has a little section on their site which explains how you can check if you can fill in fields (it says for v7, but I'm sure it applies to the current version too):
Quote:
To determine if you can fill a PDF form on screen, check to see if it contains interactive form fields and check for restrictive security settings. (For more information about PDF security settings, see the Acrobat or Adobe Reader Help.)
To determine whether a form contains interactive form fields:
1. Open the form in an Acrobat product.
2. Select the Hand tool, and then drag it over fields in the PDF form. The form contains form fields if the Hand tool changes to either of the following icons:
-- An I-beam (the Text Select tool, which you use to type text in a text field).
-- A pointing hand (which you use to select an option or to check a box)
3. In Acrobat 3D, Acrobat 7, or Adobe Reader 7, click Highlight Fields and/or Highlight Required Fields in the Document Message Bar. Interactive fields will display a colored fill.
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Full document is
here.
If that fails, it appears there is an online service that lets you type over a PDF document by uploading it online which should work for you:
http://www.pdffiller.com/