If a form has been created using Adobe Acrobat (or other compatible software), which provides blank fields for completion by the user, you can use Adobe Reader to complete the form (and to save the result).
However many offices don't have Adobe Acrobat (or similar). They either scan an existing paper form (and then save it as a PDF) or create a form using word-processing or desktop publishing software (such as Word or Publisher) and save that as a PDF.
Form produced by that second method are simply 'photographs' of the page that were created. There are no editable fields in them. (So you can't use Adobe Reader to complete them).
You'll have to print the form out and complete it by hand, scan the result into a suitable program and then save it as a PDF. (If you've not already got the means to save to PDF, you'll need something like this:
http://www.cutepdf.co...ts/CutePDF/writer.asp ).
Easier still, print it out, complete it by hand, and use good old-fashioned snail mail to return it!
Chris