Actually, Acrobat Reader allows you to edit any .pdf with form fields. However, it won't let you save the changes, only print them.
Though there is a way to make a pdf editable/saveable even when using Acrobat Reader, it's rarely done because it takes a program that costs several thousand dollars.
If you're looking for an SR4 character sheet, I've added form fields to mintcar's SR4 character sheets. And tirsales made them editable/saveable. You can download the electonic form for either the Awakened or Mundane version from
Dashifen's site.
And as Casazil mentioned, I believe somebody also made the official SR4 charactersheet into a form, though i can't remember the link for that one.
If you're looking for an SR3 character sheet, you may be out of luck. At least, I can't think of any off the top of my head.