The document is either an interactive form, in that case you should be able to fill it out using either Adobe Reader or Adobe Acrobat. You can usually tell if you are dealing with such a form if you have a purple bar at the top of the document that informs you about the form fields.
If you don't have form fields, select the "Sign" pane on the right side, and look for the "Add Text" option. This allows you to type into the space left for your information on the form.
Once you've filled out the form, you should be able to save it in Reader XI or any version of Acrobat. If you have an older version of Acrobat, you can only save the modified form if the form author saved it as a file with extended Reader rights.
Now just attach the filled out PDF form to an email and send it back.
If you cannot fill in the form electronically, you would have to print the form and use a pen to fill in the form, and then return the filled out paper form.
Karl Heinz Kremer
PDF Acrobatics Without a Net
PDF Software Development, Training and More...
http://www.khkonsulting.com