I am still expecting an answer in case there is a pdf viewer with that option, but for the moment I can confirm the
pdfnup command, which can be integrated to Thunar custom actions and thus operate through a GUI.
pdfnup terminal command (from here) in Linux Mint Xfce it said the needed tool was not installed and recommended
texlive-extra-utils. After installing that, it still gave an error complaining of some missing LaTex package. I found here that the error can be fixed by installing
texlive-latex-recommended. After installing that it all went fine.
So, to use in terminal: open terminal in the location of your.pdf and run
Or in Thunar custom actions, add the command
pdfnup %f. Appearance conditions: check "other files"
and restrict to pdf under "file pattern".
This will create a new file called your-nup.pdf where two pages from before will be integrated on one single page from left to right.
If you need four per page, 2x2 (not all four from left to right, but 1-2 above and 3-4 below on the same sheet), use the command
pdfnup --nup 2x2 %f (indicated in a comment under the answer linked above).