To ensure that when you download a PDF e-Guide it is available offline on your iPhone or iPad, you'll need a PDF reader app, such as iBooks, Kindle or Adobe Reader.
If you don't already have one of these installed, you will need to download and install one of these from the App Store. When you download a PDF E-Guide via Safari, all that happens is the document is opened in Quick View.
At this point, your guide is not actually saved anywhere. To save the document in to a compatible app you can tap and hold the open file to get the 'Open In' options (found at the top right of your screen.)
Your compatible app will be listed as an option, select this as a location; this is where your file will be stored for future use.
If you are opening your e-Guide with Kindle you can find your guide stored under 'Docs'.