I have used plain copy machine paper (20#) and while it does work, it's not the easist to work with for two reasons. One is that it is more difficult to tear, the other is that you can't see through as easily as other papers. But it was what I had available and for most of the PP projects I worked on it was OK.
Then I made a mini-lone star that was PP and the regular copy paper was almost impossible to work with with those teeny tiny pieces, plus the method of the lone star PP is a little different as first you sew the rows which do not overlap each other, then you fold the paper so the two rows meet to sew the rows together. The bulk of the 20# paper plus the 1/4" seam of the paper was difficult to manage!!
So recently I splurged ... I spotted by accident a package of Carol Doaks foundation paper and thought I'd try to to see if it was worth it. I honestly do not remember the price, but whatever it cost ... I would gladly pay it again instead of fighting with 20# copy paper!! It's thinner and easier to see through and tears away nicely.
I still use a smaller stitch though, even with the Carol Doaks paper.