Depends how the pdf form was created. Forms on the US IRS website can be saved and filled out using Adobe Reader (free) or even Apple's Preview. To use Preview, choose the text tool (uppercase "A") then click on the line, box you want to edit and type away (you can also tab through the input lines).
However, completing pdf forms is usually NOT this easy. IRS was just user-friendly enough to create pdf forms that could be filled out this way.