PENN HILLS, PA. -- Once again, the voters have figured out a paradox that puzzles the ''experts'' on the Potomac.

While Washington is divided between those who think Mikhail Gorbachev is a dynamic leader who has opened the doorway to disarmament and those who believe the Soviet leader has made President Reagan his dupe, everyday Americans are taking a far more common-sense view of last week's summit.

Voters in this middle-class Pittsburgh suburb are glad to see the superpowers talking and finding ways to eliminate even a few of their weapons of mass destruction. But they need a lot more tangible proof before they will believe that the communist leopard has changed its spots.

Two days of postsummit interviewing in a ticket-splitting precinct here reveals a fascinating paradox: the more successful Gorbachev has been in presenting himself as a TV-conscious, public-relations-smart, modern-day politician, the more he runs into the same credibility problems American candidates face from a skeptical, even cynical, public.

''It's nice he came over,'' said 57-year-old machinist Dominic Monfredi. ''Maybe we can get along a little better. But basically we don't trust them and they don't trust us.''

Down the street, retired Westinghouse worker Thomas Kelly also has his doubts. ''Gorbachev comes across as a pretty nice fellow,'' he said, ''but the people behind him are the same old regime. Russia's had a plan for a long, long time to get our missiles out of Europe and keep all their divisions in East Germany and Poland. . . so they'll have the upper hand.''

That tone of wariness was expressed also by some of the baby-boomers whose political outlook was shaped more by Vietnam than the Berlin airlift. Parole officer David Flick, 39, said he thought the summit was ''brilliant public relations on both sides . . . but didn't really do anything to change the balance of power.''

''With Gorbachev,'' said his wife, Gloria, a hospital billing clerk, ''there's more possibility of cooperation, but I don't feel the world is any safer.''

One reason for statements like hers is that Americans have come to see that communism is not the only -- or perhaps the greatest -- danger in the world. As American tourists and businessmen travel to one communist super-state, China, and relations warm with the other, Russia, Third World violence and terrorism, along with conflicts in such historically volatile trouble spots as the Middle East and the Persian Gulf, have become the center of concern.

Washington Post reporter Haynes Johnson found that shift in presummit interviews in Mason City, Iowa. Its former mayor, Republican Thomas E. Jolas, said: ''There's a different force at work in the world today, and that force is radicalism. It bothers people a lot more than the threat of communism. The Russians are not as radical as the radicals we see. . . . I'm talking about people like Khomeini and terrorism. . . . That's a real threat to order.''

The same thing could be heard here. ''I think Iran and the Gulf are a potential disaster for us,'' said Edwin Hoover, who helps arrange financing for leases of heavy industrial equipment. ''It's a lucky break for us the Russians don't want to get involved that much.''

Historians will note the irony in the fact that after seven years of intensive military buildup, aimed at countering the perceived Soviet threat, Americans no longer see the Russians as the greatest danger in the world.

As David Flick put it, ''If we ever get into a war, it won't be with Russia. It will be another Vietnam or Afghanistan. That's the only kind of war there will be. No one is going to use nuclear weapons.''

Whatever doubts remain about Gorbachev's motives, the Washington summit and the prospect of a return engagement next year in Moscow have helped rescue Reagan's presidency from incipient lame-duckism. The postsummit Washington Post-ABC News poll showed approval ratings for Reagan up.

For now at least, the summit has largely eclipsed the embarrassment of the Iran-contra affair, which dominated news coverage of the White House for nine of the last 13 months.

Responding to a question, Gloria Flick said the arms sales to the ayatollah had lowered her estimation of Reagan. ''He looked so foolish,'' she said, ''and it was obvious he wasn't telling the truth.''

''But after the summit,'' her husband interjected, ''who the hell is going to remember that?'' Given the skepticism of the public, the best way for any politician to recover from a setback is to erase it from public consciousness. That was the gift Gorbachev left behind for Reagan, and a welcome Christmas present it is.