Okular is more of a viewer than an editor. You can add things to pdfs in Okular but not really edit it. It is however excellent for extracting text, images, and tables into a TextDocument. It can also be used for displaying Presentation files and has a Presentation option when displaying pdfs or Presentation files. In terms of splitting and merging you can download the free package, pdf arranger. Just open a terminal and enter:
sudo apt install pdfarranger
When it comes to editing you have two proprietary (paid for) choices, Master pdf Editor or pdf Studio Pro. Having read reviews on alternative to I went with pdf Studio Pro - this is available for all platforms, Windows, Mac and (GNU/)Linux.
It costs about a fifth of what you would pay for Adobe Acrobat and unlike Adobe which is on a rental plan where you have to pay for extras, pdf Studio Pro has it all. I purchased it whilst working from home during lockdown and it was excellent when I had to modify a past Exam paper that was being used for Mock Exam for AS Physics. If you download the Unofficial Manual for Zorin 17 Core that I authored I devoted a whole section on pdf software under useful applications.