The biggest issue is that Kobo doesn’t allow the user to interact with a PDF document’s text. Instead, what we have is a PDF viewer with some basic tools to fit text to width, fit text to page and to switch either to landscape or portrait mode. It is possible to Navigate table of contents if the PDF supports the feature. Pinch to zoom is a mess but there is the option to incrementally zoom-in via a size scaling bar. There is no option to tap to scroll down a page; to scroll down the user needs to drag downwards. The method is frustrating and slow, as you need to wait for the e-ink display to catch-up with your movements. Again, the biggest issue, other than the slow and frustrating navigation of PDF documents, is that you can’t highlight text, write notes or look-up definitions.
|Double tap to zoom, tap to scroll & turning document page (Kindle Oasis)|
The Kindle's PDF software allows the user to view a page in portrait or landscape mode, similar to the Kobo Aura One. The user also has the option to change margin size (there are three options). Pinch to zoom is not the best but still better than Kobo’s version of the feature. The best way to zoom-in, in portrait mode, is to double tap on a page. Double tapping removes the margins and, depending on the page size of the PDF, renders text legible in portrait mode. Further, tap to scroll is supported, whether the document is zoomed-in or not. 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’ve created an animated GIF (see above) that demonstrates how to zoom-in to remove excessive margins, scroll down a page and then turn a page using the buttons.
One issue that is bothersome is the Kindle's activation of a full page refresh when scrolling or turning a page (with e-books, it is possible to turn-off a full page refresh with every page turn). Finally, all Kindle e-readers allow the user to highlight text, take notes, look-up words in the dictionary, Wikipedia (if on-line) and translate words (of course, this depends on the nature of the PDF document. For example, none of these features are supported with a scanned PDF document).
Update: I've written a correction to this post here.