Editing PDF Files Using Open Source Software

Here is a workflow that allows quite fancy altering of PDF files using only open source software. I've only used it to add overlays, so your mileage may vary.

1. Use pdf2svg to convert the PDF to SVG. Since PDF may contain multiple pages, each individual page must be converted to its own SVG file, but you only need to convert the pages you will edit. If you are not editing a page, you don't need to convert it but keep the original PDF file; we will use it later.
2. These SVG files from (1) can be edited natively in Inkscape. Use "Save as..." and save the result using PDF via Cairo (*.pdf). I did not convert text into path.
3. Follow the suggestions in How to concatenate PDFs without pain; I used the last pdfpages .tex approach that allows me to assemble from various PDF files. This requires pdfLaTeX, which comes with a TeX distribution, e.g. TeX Live. The .tex file must be processed through pdflatex and not other variants of latex command.

The SVG output from pdf2svg is a bit clunky. Not sure if that's because fonts are embedded. The PDF output saved via Cairo is also a bit clunky, but the save is quick. Finally, pdfLaTeX runs fairly quickly and produces an optimized file comparable in size to the original.