Converting, editing and compressing images is very easy using gThumb. It will provide you with basic editing tools like, colour correction, special effects, cropping etc., and using the save image as option will provide you with conversion and compression options.

The Eye of GNOME is another great tool for converting and compressing images using the save as option.

For PDF, I use GIMP. It may be overkill for kost users, but it is a pretty useful tool for basoc editing as well, like adding text overlay, and other options covered in gThumb.

Pandoc wad gonna be my recommendation for anything document based, but like ImageMagick, it's command line, that sent me looking. Looks like https://panwriter.com/ is a good front end for it, probably gonna give it a try myself.

For documents, nothing quite like pandoc, I would be surprised if it's not what most of those sketchy websites use on the backend anyway.

Xnconvert for some of these tasks

I use pdfarranger, it can do everything I need. Not sure it can do everything you need so it's best to just test it. Not sure it can compress for example.

for images, i use squoosh.app. it is a webapplication and not a desktop app. it can compress, convert & resize various image types. it is opensource and running offline (locally on your computer). github

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/

Converseen