The 27-year-old, capped once by Portugal, has agreed a three-year contract after joining for an undisclosed fee from Legia Warsaw.
Sa made a flying visit to sign for the Championship club on Monday afternoon, and has flown back home for the birth of his new baby.
He will return to England next weekend, to travell with the squad for their pre-season tour to Thailand on Sunday, July 12.
Royals manager Steve Clarke told the club website: “The striking department was one area of the team that everyone agreed needed strengthening this season.
“Obviously Orlando’s goal record is really impressive but, for me, I was most impressed by his answer to my question about why he wanted to come to play for Reading in the Championship.
“Immediately he replied ‘I want to play in the Premier League again and Reading can take me there’.
“He is motivated to prove that his previous spell in England with Fulham came too early in his career. He is a strong player who leads the line well and has an eye for a goal. I’m sure he will be a big asset to the squad this season.”