Asslam o Alikum!
Please update android builder it doesn’t build app bigger than 30MB. Means can add Assets Upto 20MB. My App Size approximately 180MB. When i click on builed Apk, a Dialog Box appears and loading start. When 100% Loading Ok , It’s Stuck on 100% and download button not appears. So give such an update that atleast 300MB app Can be developed.
are you serious 300mb?
Yes I Use 10 Pdf Books In My App
Almost 99.99% users will prefer to Download small size of app. And personally i will not Download 300 MB app for pdf.
You should seperate pdf in chapters and Load dynamic using internet after install the app.
You can use Firebase Database for free to show Listview of your books after that every chapters in book and Load pdf of that chapter in another screen using url of the chapter.
By using this method the size of your app will be almost 12 mb
I have No Idea How To Use Fire Base 
Dear @Abdullah_Bukhari , you can use any database and upload your pdf or get & set direct link in application. Then the size of application small.