It can't be done easily. Even with the full version of Adobe, if the document is locked you can't edit it, that's the idea of PDF files. If the user deems it necessary for you to enter data using the computer they will release access to those fields in the document only, the government Pensions Service issue PDF's that can be filled in like that.
I have pasted the PDF document into a Word document, it then appears as an image, then text can be added using the drawing tools, I then save it again as a PDF, which I can do because the office suite I use can do that. Unfortunately you do get a loss of resolution.