Memrise is good. There are flashcards for pretty much anything that you would want to study. I am using it to help me study pharmacy and chinese. (I have a few other suggestions specifically for Chinese if you are interested.)
A pdf reader is a very good suggestion. Highlight/annotate if you find that useful. I sometimes take screenshots of particular bits that I want a quick reference to later. I use Foxit for straight reading and Mobile Office Pro for editing/completing workbooks. (Not free, search for cracked version.)
I use Fbreader for distraction-free and comfortable reading of mobi/epub books.
Get a note application. I personally use Colour Note and have a full screen note widget on one of my 2 home screens, so its always available for me to see and edit or switch between notes. I think it is beneficial to try to be more active (taking notes) rather than just reading passively. Obviously, hand writing in a physical note book would be even better.
Get podcasts to do with your interests (or rip the audio from youtubes) to listen to when staring at your screen isnt an option.
To give more specific advice, we would need to know your field of interest. Look for online communities relevant to your interests and then see what apps they suggest. Always look for cracked paid apps. I search onhax, filetram, torrent sites and use 4sharepremium app. Always use common sense when installing cracked apps. Ive seen a few that ask odd questions when installing. Delete those instantly. Apps from reputable piracy operations are usually fine. Any other suggestions? Use AdAway if you are using free apps from the play store and Xprivacy to manage permissions of all apps. Saves battery, privacy and distraction.