Not open source, but highly trusted for over a decade now, and they provide Linux builds. XNViewMP is going to save your life as far as batch converting images goes, with reliable metadata carryover, and insanely easy GUI. Imagemagick in comparison is like building a rocket by yourself and launching it into space, with no guarantees.
For PDF, I am not sure. I like PDF X-Change Editor on Windows, and it is lifetime offline licensed, no subscriptions. PDF Arranger has good reviews for Linux, it seems. alternativeto.net/software/pdf-arranger/about/