I hate to be the voice of dissent. But you
can edit .pdf's.
But only if the .pdf maker was doing it right. Most people "lock" .pdf files so they cannot be edited, but you can set fields that are editable. If your document is editable you should be able to click in the field and you will get a cursor.
The reason I know this is that a lot of people make character sheets for rpgs this way.
For Example Click here. <- This link shows field editable .pdf file. If your document is not like this, then you are SOL, sorry.
Otherwise the best thing to do is print is, if they have given rights to print the document, and fill it out then scan it back in or snail mail it.