so seems like both these cant be used to edit/write on PDF unless i convert those first to correct format and then back to PDF. So why is that Okular can open and show and actually fill if there is containers that can be filled but not as plain PDF text
Display of PDF is a ISO standard.
But the editing is not standarised as far as I recall. (May have changed since I last checked)
Which might be why editing is not implemented.
I am not sure I understand what you mean by “convert those first to correct format”.
Okular is a PDF viewer, Calligra is an office suite without a PDF editor. PDF was not really designed to be an easily editable format but a format to replicate the contents of a page, i.e., images and text (including font faces, weight, etc.) and their exact positions on a page.
By “containers that can be filled”, do you mean PDF forms? My best guess is that Adobe saw this as a good usecase for PDF as it meant you could distribute PDF documents with empty spaces, checkboxes, etc. that could be printed and then filled out with a pen or filled out on-screen and then either printed or sent to somebody electronically. Which made companies pay for Acrobat to create these fancy PDF documents. And for viewer applications like Okular (or Evince on Gnome), it probably made sense to implement this part of the specification since it is such a common usecase.
If you need more advanced options, you can take a look at LibreOffice Draw, which can open PDF files and lets you add text or graphics to a document.