Save and mark up PDFs on your iPhone, iPad or iPod touch with the Books app. I use iBooks to save my PDFs to. Here is a short explanation that teaches you how to do this.
How to Save a PDF to Your iPhone or iPad
How to find your downloads on an Android device
It is best to use a PDF reader when downloading PDFs. Adobe Reader is free and one of the best.
You might want to use a file manager on your android phone. They make it super simple to find files you have already downloaded.
How to download and open a PDF document on an Android phone