Load a full-size web site like cnn.com on a Mini, Nexus 7, and Kindle Fire HD. You will need to rotate to landscape and/or zoom to read and operate them comfortably. In portrait mode, when viewing a web site scaled to fit horizontally, and assuming a 1" wide character to keep things simple, the Mini obviously has 163 pixels in which to render that character. Not so obviously, the Nexus 7 and Kindle Fire HD have only about 170 pixels to render that same character. This is due to the 7" 16:10 tablets being only about 79% the width of the 7.85" 4:3 Mini, so that 1" wide character on the Mini is only .79" on the other tablets. Being smaller also means you have to squint more. Things improve in landscape mode, but the Mini gets an advantage from being able to display more vertically; IOW, it sucks less in landscape mode than the other tablets.
For larger text, like the default size in iBooks, the Mini looks fine to me. The real problem is with rendering insanely small text as in the scenario described above, and I would expect PDFs would be another problem area for all these small, low resolution tablets, but I didn't test that. In another thread, I talked about "pinching in" to shrink cnn.com down to Mini size on a Retina iPad, and at the 7.85" 4:3 Mini size, the iPad's 264 PPI still looked pretty darn good on the scaled-down web page. A 7" 16:10 tablet would need 264/.79 = 334 PPI to look as good at rendering web pages at their standard size in portrait mode. The Retina Mini will no doubt be 326 PPI like the iPhone/Touch and look fantastic; the 7" tablets would need 326/.79 = 413 PPI to be as sharp. They'll likely be considerably less than that though and look fine.
In summary, 216 > 163 isn't the whole story.