If you scan a form onto a computer, it won't behave as a form by default. This is because the computer is just storing a picture of the form, and doesn't see it as a form.
To make a scanned form into an electronic version, you will need some software that lets you define the regions of the form that can be filled in by the user. the full version of Adobe Acrobat lets you do this, as do the less expensive programs CutePDF Professional
Basically, you scan your document into a pdf format and use the aforementioned software to define which regions on the form should have typable text, checkboxes, etc., then you re-save it as a fillable pdf.
This works well, I do this with all my rental applications, etc.
There is a little bit of learning required to do this, but if you spend some time with the program and its helpfile you'll be creating forms with the best of them!