Gimp imports and converts a PDF as a bitmap, then exports as a bitmap losing text as editable text. Look at the file size, much larger than expected.
The best tool is Scribus, but there is a learning curve. If it is a bit of simple editing (such as adding signature) try
1. Burst the PDF into pages, PDFChain is a good bet, it is in the 'buntu repo. (a front-end for PDFTK)
2. Edit the required pages using Inkscape, use the latest version, 1.2 (not the 0.9 from the repo) I use the appimage from here: https://inkscape.org/release/1.2.1/gnulinux/ but there is also a PPA
You can make your signature in Gimp, a png is ok to import into Inkscape and put on page.
3. Join all the pages back together using PDFChain.
Alternative: LibreOffice (LO) might do it as well, sometimes the font gets screwed up. You just have to try it and see.
The best tool is Scribus, but there is a learning curve. If it is a bit of simple editing (such as adding signature) try
1. Burst the PDF into pages, PDFChain is a good bet, it is in the 'buntu repo. (a front-end for PDFTK)
2. Edit the required pages using Inkscape, use the latest version, 1.2 (not the 0.9 from the repo) I use the appimage from here: https://inkscape.org/release/1.2.1/gnulinux/ but there is also a PPA
You can make your signature in Gimp, a png is ok to import into Inkscape and put on page.
3. Join all the pages back together using PDFChain.
Alternative: LibreOffice (LO) might do it as well, sometimes the font gets screwed up. You just have to try it and see.