Running Mavericks, if I open a single-page PDF document in Preview and size the window to 822+ pixels tall (800+ viewable document area) then the vertical scroll bar disappears (I have it set to always show) and the normal two-finger scrolling does not work in the up-and-down direction. I can still flick two fingers to scroll, and if I scroll at a diagonal angle it will work, but it wont scroll normally up and down. Resizing the window to less than 822 pixels tall solves the problem, making the scroll bar reappear and the trackpad scroll gesture works properly. Multi-page PDFs, as well as non-PDF files, scroll just fine. It is only single-page PDFs, and only when the window shows at least 800 vertical pixels of the document, that Preview stops vertical scrolling. Does anyone else experience this?