The South Americans dominated an at-times scrappy match against a spirited Indonesia team who were energised by a rapturous crowd of 56,000 which cheered every pass, throw and foray into the Argentine half.
The visitors had the best of the early action, with Nahuel Molina and Nicolas Gonzalez both denied and Facundo Buonanotte squandering a chance after skipping around the advancing keeper.
They finally made their breakthrough in the 38th minute when Paredes found space and blasted into the top corner from 30 metres.
Argentina doubled their lead 10 minutes after the break when an unmarked Romero converted with ease from a Giovani Lo Celso corner.
There was no hiding the disappointment in Indonesia over Lionel Messi's absence, despite his part in Argentina's 2-0 win over Australia in Beijing on Thursday, when he netted in 79 seconds, his fastest international goal.
Dressed overwhelmingly in red and waving national flags, the fans in the Gelora Bung Karno Stadium still chanted "Messi" as the team walked on to the pitch.
Indonesia defended in numbers to try to stem the onslaught and created a few chances in the second half, testing keeper Emi Martinez with a succession of long throws into a crowded box.
But they stumbled on the attack and struggled to contain Alejandro Garnacho, one of six second-half substitutions, who almost made it three in stoppage time.
"I'm quite satisfied with my team's performance, including Garnacho," Argentina coach Lionel Scaloni said. "I hope the young players can further develop."
The match was the only bright spot in a torrid eight months for football in Indonesia, during which 135 fans were killed in one of the world's worst stadium stampedes.
The Muslim-majority country was stripped of its right to stage the under-20 World Cup in March over domestic resistance to Israel's participation, and replaced as hosts by Argentina.
Indonesia's South Korean coach Shin Tae-yong said the match was a confidence boost ahead of next year's Asian Cup, while the country's president, Joko Widodo, said he had feared the team would be trounced.
He described their second-half performance as "extraordinary".
"Argentina are ranked first and we're 149th, but we only conceded two goals," he said. "This is a big experience for us."
