If I am not mistaken, you may be in search of the common word "freeloader", "a person who takes advantage of others' generosity without giving anything in return." While it's informal, it is far more common than most other applicable terms. A synonym of beggar might suit your needs though, perhaps mendicant (more formal and doubles as an adjective, but less commonly understood), panhandler (informal), etc.
The fact is that if one fits this description, he can't be too picky about what people call him.
Freeloader: "a person who takes advantage of others' generosity without giving anything in return."
Beggar:(1st sense) "a person, typically a homeless one, who lives by asking for money or food."
Mendicant: adjective - "Given to begging" noun - "Beggar"
Panhandling: "Begging or panhandling is the practice of imploring others to grant a favor, often a gift of money, with little or no expectation of reciprocation. Beggars may be found in public places such as transport routes, urban parks, and near busy markets."