When you download a .PDF file to your iOS device, like an iPad or iPhone, it really only opens the file for you to read at that time. There is a way, however, to Save the file for later viewing. These instructions were written for using the Safari browser, but it should be similar in other browsers.
After you have clicked on the download link, usually either sent to you in an e-mail or a link you’ve clicked on a website, the .PDF file will open up in your browser. These instructions were written for using the Safari browser, but it should be similar in other browsers. If you tap on the screen near the top of the file, you’ll see the option to Open in iBooks.
Once you choose to open in iBooks, the file will be saved in your iBooks library.
To continue reading the file at a later time, simply open up the iBooks App and you’ll see it in your library. You can also choose Open in… to open in another eReader app.