If you want to be able to read anything on a device in a direct sunlight your only choice is an ereader using electronic ink. Here is an example, outdoors on a sunny day with the sun blasting directly at the screen (try that with a tablet or your phone):
From my own experience 9.7" screen size is the minimum you need to read technical literature. I tried with 8" hi-res screen but it was just too small. I made an almost complete switch from paper to the ereader and I read books mainly in the PDF format from sources such as Manning, O'Reilly, Packt, Safari and books that I couldn't find anywhere and scanned myself.
You have quite a choice these days. In 2012 I started to use bespoke Linux based Onyx Boox M92 and then in 2014 upgraded to Android based Onyx Boox M96, both 9.7" electronic ink ereaders. At present there are newer N96 (dual touch) and N96ML (touch + frontlight) 9.7" Android devices and 13.3" Onyx Boox Max. There is also 13.3" Sony DPT-S1 but it runs custom Linux (limited software choice).
You don't list these as requirements but there are some things of secondary importance after the screen that still quite matter to me in the reader I have:
- very long battery life since the screen doesn't use it once the page is displayed
- ability to install Android apps (dictionary, cloud client, VPN client etc.)
- connectivity (wifi, bluetooth)
- external storage (microSD cards)
- and finally ability to scribble/highlight/erase as I would do in a normal book.
One potential disadvantage of ereaders - they are not very powerful. This can be an issue especially with scanned documents when turning a page can take up to 3 seconds (depending on resolution, complexity, etc.), which renders skimming through such documents impossible.
Below is an example screenshot from my ereader with a table and some doodles, you can zoom in the document as much as you like:
You can find loads of information on ereaders on MobileRead forums.