Some comments...
I download most datasheets so I have a local copy, and more recently I fairly religiously download them into PDF Expert* on an iPad. This is such a convenient way to store datasheets.
Relying on always having a downloadable version is risky, occasionally they just disappear from the internet, or sometimes you don't quite use the right Googling. Sometimes you might not have internet access, pretty much most days I find myself reading a datasheet on a tube ride.
All too often, Google results point to useless online viewable versions of the PDF, or worse, take you to a series of rabbit holes with no datasheet at all. Having a local copy saves you going round in circles.
I still look online anyway on occasion even if I have a local copy, frequently there are newer versions of datasheets or new errata to download.
I almost always print out the pinout page(s) for each project/design and annotate them with my own notes using a good old fashioned pen.
Until about three years ago, I had a reasonably large collection of databooks and technical references, filling 7m of full height bookshelves. Since I'd moved to using a tablet for a reading medium a couple of years earlier, I found I hardly ever referred to physical books anymore, and most of the physical texts I had were woefully out of date and no longer useful anyway (software related texts are especially prone to this). I threw out 85% of my books as a result, keeping only those that had real use or had sentimental value.
Here's one caveat to relying on PDFs or other electronic formats for documentation: sometimes, the electronic format has been rendered really badly, so diagrams are unreadable or unclear. Kindle, for example, is really bad for that in reference texts, but worse I find the reading layout itself and its page reformat on the fly to be very unnatural. At least PDF maintains the original format.
* Apple's iBooks is crap because once you get a PDF into it you can't get it out again, and when you "backup" your device, when you need to restore or move devices, half of the PDFs disappear, and those that remain have their titles reverted back to the original file name. Somehow, Apple get away with such crap.