Do you absolutely want it to be a GUI? If not, there're many great command line tools, e.g.
mutool (part of
mupdf),
cpdf,
sejda-console (the last free release is
3.2.85),
qpdf, on some distros the good old
pdftk or its newer port to Java
pdftk-java may be available.
A good GUI tool is
Master PDF Editor, but the free (as in beer) version has limited functionality, albeit older free versions (3.x) were not so limited in scope and still
can be downloaded.
Free version of
Sejda PDF Desktop imposes some daily and size limits.
The venerable
PDFedit still may be available on older distros. It requires Qt3 and is rather basic than powerful. Probably won't cope with newer and complex PDF files.
If you're ready to pay, there's also
PDF Studio. Cannot really comment on this as I never used it.
Another option is to install Wine and run a Windows PDF editor.
Or you may use some online PDF edit service if you're OK with uploading your files to them.