Putting flyers on peoples doors or property is considered to be "canvassing" and not soliciting.
However some cities and towns may have ordinances against putting flyers on peoples doors but most places do not.
A lot of people have put flyers on peoples doors without any issue and the worst thing that would happen is someone tells you to never do it again.
But you can put flyers on peoples doors in most places without any issues and it's a good way to get business as long as you put up professional looking flyers.
However do not put flyers on peoples mailboxes or in the mailboxes as that could actually get you fined for messing with postal property.
Although people own mailboxes they are also considered to be postal property.