In a stash comment I just noted, that I deleted another stash entry because it linked to a PDF full of blank pages.
I just opened another such PDF I'd earlier downloaded from the same site
(it hadn't been the first time!!)
but opened it in Acrobat Reader 7, not in Preview.app.
...Oh.
No problem... invisible ink, apparently. Whoops... Er... Ow... I guess I was just using the wrong program...