No, sites can't bring down your PR by linking to you. Otherwise, every one with a PR0 site could link to Google and knock it down to PR0 overnight.
NO , this is a myth and i don't know who mad eit up but itcn is right, it doesn't affect your PR at all
"Is my PR affected if site having lower pagerank than mine links to my site" I don't understand, do you mean affected in positive or negative way?