[BoldlyDubious]

The DR-900 offer the possibility to zoom to (few) preset levels, or to adapt the zoom level automatically to fully exploit the width OR the height of the display. If the document had the exact aspect ratio of the screen, the latter two would have the same effect; in practice, you usually have to choose. If you want the print to be as big as possible without having (horror!) to pan to read the end of every single line of text, you will usually choose "fit to width", considering that print formats are "taller and slimmer" than the 4:3 display. (By the way, all of this works both in portrait and landscape mode.)

If you use "fit to width", you will have to pan manually (by dragging your finger) when you reach the end of the first part of each page, in order to be able to read the second part. In fact (first, lesser, problem) the current firmware of the DR-900 is not smart enough to know that when you press "page forward" after having read half of a page, what you really want is the second part of that same page, NOT the first part of the following page.

The second problem, the real one, is that the current firmware does NOT detect white borders at the edges of the pages. So, if you choose "fit to width", the whole width of the document, including the white margins, is zoomed to fit the width of the display. Which is usually what you don't want, as you want to read the text and not examine the white margins. What you usually end up with in this way is a print that is too small, and a lot of useless white space at the sides. I must add that, for a device that most buyers probably preferred to 6" models precisely for its capability to manage pdf files, this is a major omission.

In the end, if you want to use all of the available width for text, you have to do one of the following:

1: select, from the available zoom levels, the one that is closer to the correct level (i.e., the one which makes a line of text fill all the width of the screen). Then, by panning with your finger, you perform lateral centering of the first page on the screen (some practice is needed to get this right at the first attempt). Thankfully, then this centering is retained for the following pages, so you only have to pan vertically to see the second part of each page.
The problem with this solution is that the zoom levels are too few, and you usually end up with some white space nonetheless. Moreover, if you are not careful when you pan a page to read the second half, you mess up the lateral centering and have to readjust it. With practice, everything becomes almost automatic but it certainly detracts from the naturalness of reading, which is why I chose the following, alternative solution.

2: pre-crop the document, eliminating all margins, before loading it on the device. In this way reading is a breeze, as you don't have to adjust anything. You simply choose the 100% zoom level, and every page fills the screen in the best possible way (of course, you still get some white space if your document hasn't a 4:3 aspect ratio, but the result is good nonetheless thanks to the big 9" display). You don't have to pan at all.
As I said, I found a fantastic little program called pdfcrop that lets you do the cropping easily. (It's a Linux program because I use Linux, but I suppose there are alternatives for Windows too.)

To know if you are comfortable with reading scientific articles (typically formatted as A4 or Letter pages) on a 9" screen, the best way is to make a photocopy zoomed out so that the result has the correct dimension, and try to read it. If you like it, you will also like the electrophoretic screen, which I find remarkably similar to (greyish, but not annoyingly so) paper.


To conclude: I'm talking about the Asus DR-900 because I own one; I expect that similar problems can arise while using pdf files with the Kindle DX.