AC offers this "document validation" service which is entirely voluntary and allows for uploading of a PCR test. ArriveCAN however neither requires nor allows the uploading of a PCR test.
For my recent AC FRA-YUL flight, I uploaded the pdf version of my EU vaccination certificate into ArriveCAN, and it was validated within minutes (status goes to I). At FRA departure gate, my ArriveCAN status was checked but not my PCR test. I did not use the AC validation service.
At YUL, border agent wanted to see my PCR test but not my ArriveCAN - I assume scanning my passport already showed him the relevant info.