Is it permissible for me to keep stationery and gifts from a telecommunications company that I received in large quantities due to my acquaintance with an official there, or must I return them or donate them?

Your friend is a private employee whose hand is that of a trustee (amanah), so he is not permitted to dispose of gifts except with the company's permission. If you know, or are reasonably certain, that he overstepped regulations by giving them to you, then you are not permitted to accept them. If you do accept them, you are obligated to return them whenever that becomes feasible. Donating them is not sufficient unless returning them is impossible.