I used to edit PDFs using GIMP. I would open the PDF in GIMP, make edits, export it as a
.mng file, and convert it back.
$ convert -reverse pdf.mng pdf.pdf
This would work well except all the texts were rasterized.
Update 2019: I recently tried editing PDFs using LibreOffice Draw again and was delighted to discover its PDF editing capabilities have improved. Gonna use LibreOffice Draw from now on.
Update 2020: Xournal has really improved! I’m currently using Xournal to sign PDFs