A neat little booklet, the coolest parts of which talk extensively about the methodology used for reconstructing the physical process by which the book was typeset and printed--the compositors, their failings, and so on.
On the other hand, it doesn't seem odd to me that marks from scissors--by marks we mean not snips, but rusty or pressed outlines--would be common in a 17th century book. The theory advanced in this article is that they were left there by binders.
Snipping the pages open, as you'd have to with a quarto, isn't an issue with a folio. But it seems to me that you'd still likely have a deckle edge to deal with.
It's a pain to turn pages that have deckle edges. They're floppy and soft and uneven. So as you read, you snip. And what's handy lying around to be used as a bookmark when the missus calls you to dinner?
I dunno--were the books trimmed before they were sold? I don't think so. Some might have been trimmed by the binder, or course.
Okay, taking my cold to bed now. The body has just weighed in on the amount of work its been expected to do recently.