I can confirm you can do this and get the achievement without all 8 pages.First Speed Run attempt - I got 1 page didn't work, second I went for 2 page's didn't work, my third try I went for 4 page's kamikazed into Slender then I got the run through the woods cutscene woke up in the next level and the achievement popped. It also worked on another test run with 7 pages, so I'll assume you need at least 4 to have this work.
I collected my 4 pages from The Warehouse, burning boat, tent and finally the visitor centre before diving into Slender.