I use Imagemagick for some things, but this is to recommend to other non technical users
A lot of people upload their sensitive files to online converters/compressors, and I’d like to recommend something easier
Say
- webp to PNG to jpeg
- compressing PDF, converting to and from other formats, extracting pages, unlocking, etc
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. https://alternativeto.net/software/pdf-arranger/about/
Love me some XNView / XNConvert for batch image conversion.