Two Separate Gorillas
It does depend on the layout of the book. A great many RPG books are clearly designed with the 8.5" x 11" format in mind, dominant after AD&D, and in that sort of space multiple columns, sidebars and interspersed illustrations work well: transfer that too a small screen and it looks awfully cramped, but it also means that text reflow options might struggle. In those cases the 10"+ displays are clear winners.I wouldn't want to go much smaller than 10.1" to read 8.5 x 11 PDFs.
Books with a layout better suited to a smaller page or actually designed with ereaders/tablets in mind (becoming slowly more common) naturally work pretty well on 8" or even 7" screens. There is unfortunately still a dominant trend in rulebooks that they need to look spectacular on the page, and that makes for a poor electronic experience unless the publisher has properly taken PDF viewing into account and not simply considered it to be the book on a screen instead of on paper.