How to transfer a book from PC

Hi. I don't know why, but I no longer can do this.
I have the Kindle App, and I can read books just fine with it.
I once was able to take a textbook I downloaded with my PC and transfer it the tablet.
Then I was able to make that book file readable through the Kindle App.
Unfortunately I do not know what format that file was in.
Additionally, I did once transfer a lot of comic book files to the tablet from my PC, and even created a folder for them.

Now I have a book, in pdf, mobi, and epub formats that I would like to read on my tablet, through the Kindle App.
I do not know how to do this, and I can't even find the folder that contains the books that I have purchased through Kindle.

Please help!