From my experience, Okular is more of a "mark-up" tool than it is an editor. There is no function/feature that allows you to perform editorial corrections ... like replacing a mis-typed letter, or adding a missing "newline".
As for "major" edits on a PDF form, to maintain style, positionning, etc., I haven't come across any that works.
Even LibreOffice Draw, it may be able to rebuild a printable PDF that keeps the original appearance, but does not seem to correctly handle the "editable" functionality of modifiable PDFs, à la Adobe.
The snap store offers onlyoffice-desktopeditors. For editing PDF documents it has more capabilities than any other free non-web solution that I've found. The trade-off is that the learning curve is higher.
The only real drawback I have found with OnlyOffice which I agree is probably the best replacement for MS office is that it only opens the whole suite that seems to take forever. You can't just open a writer, calc or .pfd instance which would be a lot quicker. At least not the last time I used it which has been awhile.
The latest version of Master pdf editor is free-for-trial, but it adds a watermark. OnlyOffice does not. Other than that advantage, I think it's a matter of preference. Jymm has a valid point, too, about the load time.