how to add bookmarks in a PDF document?

You can also add bookmarks in Okular.

@ muhammed

Yes, I agree with jlb.
I consider it an extraordinary application.
You can not only use it to add bookmarks.
You can even add text comments and use the comments as as bookmarks.
Or even combine bookmarks with comments.
You can also highlight text and comment (or not) the highlighted text
Some operating systems don't offer Evince as the default office document viewer but on some operating systems you can add it, and replace with it, whatever they are using as a default viewer.
Let me know if you need some more help with Evince.
I would love to help people with some details about how I have been using it at least for the last year.
B.

Thanks guys

Bubo I'll give it a try and if I have trouble I'll take you up on your offer to help -- thanks!

@ muhammed

OK. I'll keep an eye on this topic.