Legal Question in Business Law in California

Generally it is legal to hand them out, and illegal to do anything else with them. Anti-littering ordinances usually prohibit placing flyers on cars, and anti-blight ordinances as well as PUC policies prohibit placing them on utility poles and buildings. You would have to look to local ordinances for specifics.

Handing out flyers is a protected exercise in free speech under the First Amendment, but most other activity you mention (posting, placing on windshields, etc.) is restricted or prohibited by laws regarding trespass, littering, etc. and such laws have been tested in courts several times and found constitutional.

I was in the main courthouse in Los Angeles yesterday, and right outside were at least a half dozen individuals passing out leaflets offering various legal services to people coming into court. Certainly legal. However, I didn't see any handbills on windshields or the Dep't. of Water & Power's poles.