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
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.