I've been experimenting a bit with an 'ideal' page size for self-created PDFs.
I've attached the sample that seemed to work best: 8.5 cm wide x 11.4 cm tall and 0 margin. Kindle adds a bit of margin - and it appears that the content is zoomed to fit width, and centred both horisontally and vertically. Horisontal centering looks odd if you only have a short paragraph left on the 'page' (last page of my sample).
The zoom-to-fit also affects the size of the fonts - but I think I've come pretty close to a PDF page size that displays the fonts in a size that matches actual print size (in vertical view).
Since there's little margin top and bottom, I made another sample 1 cm shorter (8.5x10.4 cm). This also seems to work okay if you like a bit more margin top and bottom.
On the first page I drew a box along the edges of the 'paper', so they're 'visible' - and if you shift back and forth between page 1 and 2 on the Kindle, you can compare font sizes and 'margins'
The font is calibri and font size is 12 pt.
On a final note; if this is the way PDFs work on the Kindle DX, I fully understand why the PDF implementation on this device is considered 'basic'
It's okay for my needs, but it ain't great.