Many PDF documents are little more than photocopies of pre-existing paper forms. In my answer, above, I was assuming that the form was probably something based on a paper document.
If the form was actually created as a PDF document, with the intention of permitting some limited text input, it should be possible to add the required text - usually with a maximum of 1 line - as long as 'Text mode' has been selected in Acrobat Reader. (Click the 'T' button, above the document).
Chris
Chris