IOS
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.
Link: https://support.apple.com/en-gb/HT205751
How to Save a PDF to Your iPhone or iPad
Link: https://www.lifewire.com/save-pdf-to-iphone-or-ipad-4165490
ANDROID
How to find your downloads on an Android device
Link: https://www.businessinsider.com/where-do-downloads-go-on-android
It is best to use a PDF reader when downloading PDFs. Adobe Reader is free and one of the best.
Link: https://play.google.com/store/apps/details?id=com.adobe.reader
You might want to use a file manager on your android phone. They make it super simple to find files you have already downloaded.
Link: https://tunesgo.wondershare.com/mobile-phone/android-file-manager.html
How to download and open a PDF document on an Android phone