The pope added that people should say what they think, but "with respect".
Asked whether he hoped that President Trump would change his policies on matters such as immigration after their meeting, Francis replied: "That is a political calculus I can not afford to make. We need to find the doors that are at least partly open, go in, and talk about things we have in common and go forward, step by step", he said.
Pope Francis says the Vatican has a 2,000-case backlog in processing clerical sex abuse cases and admits criticism of the slow pace is justified.
During a news conference on board the papal plane, Francis said he would look for common ground at Wednesday's meeting - the first between Trump and the leader of the world's 1.2 billion Catholics.
"Peace is handcrafted", said Francis. "Step by step, peace is crafted each day", he added. "Friendship, esteem ... they, too, are handcrafted".
The two leaders differ greatly on immigration and climate change.
They famously had a tense exchange in February 2016, when Trump called the pope "a political person" for celebrating a Mass near the U.S. -Mexico border and Francis questioned the pope's Christianity over his support for a wall on that border.
The two men are due to meet at the Vatican on May 24 while the president is on a tour Saudi Arabia, Israel, Italy and Belgium, suggesting their previously icy relationship may have thawed.
On clergy abuse, the pope was asked about Marie Collins' resignation from the Pontifical Commission for the Protection of Minors.
Speaking to reporters while traveling home Saturday from Portugal, Francis said he would say what he thinks sincerely to Trump and listen respectfully to what Trump has to say. He revealed that the possibility of having continental tribunals (such as in Latin America) to enable the local churches to deal with abuse cases in a more effective way.
Speaking about Marie Collins's resignation from the papal commission, Francis began by praising her as "a good woman, a courageous woman" who had "explained everything" to him.
Pope Francis blesses a child upon his departure from Monte Real Air Base, in Leiria, Portugal, Saturday, May 13, 2017.
"I have never signed a grazia", the pontiff said bluntly.
"We are going ahead", Francis said Saturday. The tiles show Lucia Santos, Francisco Marto and Jacinta Marto, the Portuguese shepherd children who say they saw visions of the Virgin Mary 100 years ago.