flpsed works well for entering plain text and tags within a PDF document. flpsed is rather basic, it does not allow adding images (for example: an image of your signature), but it is very convenient for filling out a PDF document with simple text input.
$ sudo apt-get install --no-install-recommends flpsed
$ flpsed document.pdf # to edit document.pdf
gimp supports importing PDF documents and do anything with the PDF that you can do to a traditional image in gimp.
$ sudo apt-get install --no-install-recommends gimp pdftk
If your version of gimp does not support exporting as .pdf you can use the convert application which is contained in the imagemagick package.
convert
(located in the imagemagick package) to convert from a .png
to .pdf$ convert document.png document.pdf
pdftk
to concat the separated pdfs back into one PDF
document.$ pdftk document1.pdf document2.pdf document3.pdf cat output document.pdf
where document1.pdf, document2.pdf and document3.pdf are the individual .pdf files and document.pdf is the single pdf document that contains all of the individual pdf files.
flpsed allows quick and easy editing while gimp/pdftk provide a robust suite for nearly all pdf editing needs. Both approaches provide an alternative to printing documents, signing them and faxing/emailing back.