I tried with https://www.linkedin.com/in/kevinrose while not logged in and "saved as pdf" doesn't seem to be missing any lines that I can see.
As for the chat box, I don't think there's anything you can do about it except hide it before saving.
What page are you saving? Is it a long page? Adobe has a limit on a page's length that you might be running into. See https://forums.opera.com/topic/27752/opera-build-in-save-as-pdf-for-lengthy-webpage.
@qaz1111123 said in Opera doesn't open pdf:
How is it possible?
It's a known issue. Not sure how it's possible, but the PDF viewer is technically an extension (an internal one). Some change in it must be allowing the extension to interfere. It's being looked into.