Editing Tools for PDF files
- Xournal++
- LibreOffice Draw
- Inkscape
- Gimp
- and many more ...
It also depends on the PDF file itself whether one of these tools properly visualizes the original PDF file. So depending on the PDF files you might choose one tool over another. Recently I've been using Inkscape, because LibreOffice Draw tends to screw the text alignment of a PDF files sometimes. A drawback of using Inkscape is that it is only capable to open one page at a time. So in case you want to modify several pages with Inkscape, you'll have to do it one by one and use tools like PDF Arranger to merge the resulting PDFs afterwards again. Since many forms have between 2 or 3 pages it might be worth the effort.
Xournal++
Xournal++ is an open-source and cross-platform note-taking software that is fast, flexible, and functional. A modern rewrite and a more feature-rich version of the wonderful Xournal program.
You can also easily modify and create PDF files with it.
Also see comprehensive GitHub Page: https://xournalpp.github.io/
Install Xournal++
sudo add-apt-repository ppa:apandada1/xournalpp-stable
sudo apt update
sudo apt install xournalpp
Also see xournalpp installation
Inkscape
Install Inkscape
sudo apt install inkscape
PDF Arranger
Install PDF Arranger
sudo apt install pdfarranger
Merge several PDFs into one
Open PDF Arranger, press the + button, choose the pdfs, which should be merged, arrange them and then save one single pdf.
Decrease size of PDF files
Ghostscript can be used to do several things with PDF files. For example also decreasing its' size.
gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/ebook \
-dNOPAUSE -dQUIET -dBATCH -sOutputFile=output.pdf input.pdf
More details can be found here: https://www.ghostscript.com/doc/current/VectorDevices.htm#PSPDF_IN