I've long railed against use of PDFs for documentation largely on the basis that they're so painful to read online.

My views started shifting when I had access to a 9" tablet, and later an iMac Retina with a 5k screen.

What's completely sold me however is the 13.3" e-ink tablet I picked up earlier this year.

It has 1650x2200 resolution 16 shade greyscale at 207 dpi. Roughly the size of a sheet of US letter or A-4 paper. And it is delicious.

The problem with PDFs is not inherently PDFs. It's that computer screens and mobile devices simply stink for reading typeset content. Especially laptop displays.

PDFs can go wrong, and trust me, I've seen some bad ones, but they're bad for the same reason there are unreadable websites and shitty apps: designers with no knowledge of how to or interest in serving the needs of readers, and rather too much firepower at their disposal, aimed at foot.

The iMac display, when I happen to be in front of it, is wonderful (and I like others wonder why the 5k display format never took off --- it really is a sweet spot between 4K and larger devices, to say nothing of the flood of low-quality low-resolution monitors still crowding stores, online or B&M). With that display, I can view two-pages up, and have space left over for other work (a frequent requirement), as well as single-page views.

I'm not much enamoured of Adobe or what it's done with PDFs. Editability, PDF forms, and DRM are all poorly-behaved or malignant. (PDFs aren't intended as a modification format, forms ... should be addressed by other mechanisms, and DRM is cancer.) But the stock format itself does a job, does it well, and supports extensions including underlining and notetaking. These could be improved, but the fundmaentals are good.

The main problem with PDFs isn't PDF's. It's shitty device displays.

A low-quality printer manages about 300 dpi, and high-quality ranges from 600--1200 (photographic). At 200--300 dpi, e-ink is well in the "as good as consumer printers" basis for resolution. Readability improves as ambient light increases, and the experience under direct sunlight is excellent. Indoor-viewability varies --- the baseline white remains somewhat dark, and diffuse overhead lighting (flourescents) can result in glare, but that experience is no worse than with most emissive displays. Frontlights are available and work well. Colour remains a rarity, though it's not much of a price premium. Even where present it is muted, not vibrant. I can live with that.

But the really critical factor is size. I'd argue that the 8--10" range is probably the sweet spot, where reading most general works becomes viable. I read scans of older publications, including many journal and popular magazines, often scanned from copy with small fonts and quality issues, all but the worst of these are readable without requiring zoom on my 13.3" display.

Note that virtually all I have to say here applies equally to DJVU, another layout-oriented document format.

The issues I have with ePub and HTML are that they don't preserve formatting, don't offer hard pagination, often require scrolling (a horrible experience on e-ink, or frankly, on documents generally), and don't allow persistent reference by page.