I've found what is an irritant with Lastpass in Gmail. When I open a password-protected PDF in Gmail, of course Gmail prompts me to enter a password. Upon doing so, Lastpass prompts me about which Google password I want to update. The answer, of course, is none, or my next login to Gmail would fail.
So, in other words, LastPass should recognize when a password is being entered to open a PDF and either a) offer to store the password in a separate section, or b) refuse to update the Gmail password, as the password for a document is not generally concurrent with the password to the site.