The Kindle app reads these formates:
Microsoft Word (.DOC, .DOCX)
Images (.JPG, .JPEG, .GIF, .PNG, .BMP)
Kindle Format (.MOBI, .AZW)
Install the Kindle Reader for Windows on your PC and use the option 'Send to Kindle' for to send your books via WiFi on your phone to the Kindle app directly or to the Kindle server for later download to the Kindle app.
If you already have these books on you phone, you'd use ES File Explorer's (or maybe your stock file manager's) option 'Send to Kindle' which will do the same as above described.
If you don't want to use these methods and want "pasting them into the Kindle app" you've to create a folder "kindle" on the root of your (internal or external) SD card and drop the books into this folder.
They should appear on the Kindle app's book shelf