Ronaldo, 37, wants to leave Old Trafford in search of Champions League football in the coming season, only a year after rejoining the club and with another year on his contract.
Mendes discussed the availability of the No. 7 with PSG sporting director Luis Campos and club president Nasser Al-Khelaifi. The super agent is very close to both of them after having worked for many years with Campos before his arrival in the French capital this summer. He has also handled numerous contracts for his players with Al-Khelaifi, including PSG's most recent signing Vitinha, who joined from Porto.
Sources added that PSG don't feel that Ronaldo is the right signing at this moment and that the club doesn't have enough space in their wage bill to pay him.
From a sporting perspective, there are also concerns that there isn't room for him in the team, with new manager Christophe Galtier wanting to start a season with using a back three. The former Nice coach may already struggle to find ways to play Neymar, Lionel Messi and Kylian Mbappe together in his preferred formation, so a move for Ronaldo would not make sense.
PSG are not the first club to say no to Ronaldo this summer. Bayern Munich also turned down the chance to take on the former Real Madrid and Juventus star, while Chelsea manager Thomas Tuchel is not keen either, despite a meeting between Mendes and new Blues owner Todd Boehly to discuss the Portugal international's future.
In addition to Ronaldo's desire to play in the Champions League in the upcoming campaign, the six times Ballon d'Or winner is also said to be unhappy with the lack of transfer activity from United so far this summer.
However, Manchester United's new manager Erik ten Hag insisted the club wants Ronaldo to remain with the team as recently as Monday in a press conference before the Red Devils' 4-0 friendly win over Liverpool on Tuesday in Bangkok during their preseason tour.
"We are planning for Cristiano Ronaldo for the season and that's it," Ten Hag said. "I'm looking forward to working with him. I have read [that he wants to leave], but what I say is Cristiano is not for sale, he is in our plans and we want success together.
"I spoke with him before this issue came up, I had a conversation with him and I had a good talk. That is between Cristiano and me. What I can confirm is we had a really good conversation together."
Ronaldo, who has been granted time off to deal with a family issue, did not travel to Thailand as part of the tour.