Can anyone explain why a editable pdf becomes unreadable after edited with okular?
Unreadable by what?
"editable PDF" is a bit of a misnomer. PDF isn't a WYSIWYG word-document or desktop publishing format, it's a printing format, already boiled down to the point where handy things like word-wrap and such cease to exist. As such, editing them is always a bit dicey.
Everything but okular.
Sorry I used the wrong terminology. It is a fillable pdf. Something like this. Can you please fix my title?
Bug in Okular? According to Okular - Document format handlers status PDF form support is a work in progress.
Can you recommend a Linux PDF program that properly handle fillable pdf's?
There is such a thing as Adobe Reader for linux.