There are two types of form - fillable and non-fillable. If the form was not designed as fillable - entirely possible - print it and hand write it I'm afraid.
Organisations such as Citizenship and Immigration Canada are currently transitioning from non-fillable to fillable, and usually "FILLABLE FORM" on the new ones.
Acrobat Reader 5 should allow you to fill out BUT not save the entered data.
For that you need Acrobat Approval [which I have and works well]. It allows you to fill in the boxes and save the form with the data inside so you can print it later. I have Approval - download from adobe.co.uk I think, large one though - and it works well.
Adobe Acrobat full version is a waste of dosh unless you are amending a form or creating new, remember even if you are allowed change the form HMG might spot the difference and refuse it!