Free Pittsburgh City Directories for Pittsburgh, Pennsylvania (including suburbs of Beaver, Beaver Falls, Butler, Cambridge Springs, Connellsville, Franklin, Greensburg, Kittanning, Meadville, Monongahela, New Brighton, New Castle, Oil City, Rochester, Sharon, Uniontown, Washington, and Youngstown) are available at Internet Archive. 

City directories contain an alphabetized list of residents with their addresses, spouse’s names, professions, and often a reverse lookup by address. They may contain historical information about a locale and they’ll usually contain a separate business directory.

Internet Archive currently has the 1920 Pittsburgh City Directory available for free searches.