I found it useful that the Oasis’s buttons only turn to the next page of a PDF document, rather than scroll down a page; this is a good implementation as it removes any confusion between page scrolling and page turning.I mainly read PDF files in landscape mode, due to the restricted seven inch screen estate, and, consequently, didn't notice the mistake made in the above observation (also, recently, I've predominately been reading e-books). To clarify, tap to scroll or using the buttons perform the exact same function in a zoomed-in PDF document: i.e., both enable the user to scroll down a page and turn to the next one when a page is completed. Unfortunately, the implementation of this function is buggy and the device, sometimes, for no reason, zooms-in further. In addition, when scrolling to the next page, the Oasis does not jump to the top of the page. As a result, depending on the PDF file, some lines could be accidentally missed. To ensure a page-turn starts at the top of a page it then becomes necessary to drag manually upwards.
To resolve these issues with tap/button-click to scroll, the best option, in my view, is to scroll a page by dragging downwards when zoomed-in. After completing a page, again to avoid the tap to scroll function, first zoom-out with a double tap and then follow this by a tap/button-click to turn the page. In the next page, repeat the double-tap to zoom-in and drag to scroll downwards. This option might be cumbersome but it is workable and makes PDF portrait reading more stable.