I think this stated backwards, landcape being long side up. Anyway, Mini is a good technical reader in landscape, but you will need to scroll. The ipad in portrait fis ok fpr techincal reading, but it is a little small--being less that 9.7" and text page is 11+"--so it feels a little cramped. Bottom line--neither is perfect for technical reading, and buyers need to make their compromises.
landscape as in home button to the right or left side, portrait as in home button being on bottom, facetime cam up top.
The retina IS the better reader period. If you objectively look at the retina screen, it is truly remarkable when rendering high quality pdfs. The mini in landscape mode will require scrolling vertically that's why portrait orientation is much better and more natural for documents on the retina ipad - it can render everything in fine detail without the need to zoom. When you scroll, you are just scrolling down a page at a time.
I don't read pdf's in landscape orientation unless they are car brochures.