There are two ways to save a PDF to an iPad, depending on which version of iOS you're using. You can find out which version of iOS you have by opening your iPad's settings, tapping on General, then tapping on About.
iOS 10 or earlier
In iOS 10 or earlier, you can save PDFs to read offline using Apple's iBooks app. If you don't have it already then you can download it for free from the App Store.
With iBooks installed, open a PDF in Safari and tap on the open document. You should see the option to 'Open in "iBooks"'.
If that doesn't say iBooks, select 'Open in...' and you should see the option to 'Open in iBooks'.
You will then be taken to the iBooks app where a copy of the PDF will be saved. You'll be able to access this copy even when you're offline.
iOS 11 or later
If you're using iOS 11 or later, you can also save PDFs to your Files. To do this, open the PDF in Safari and click on the Share button (which looks like an up arrow coming out of a box):
You'll see a list of options. To save it to the Files app in your iPad, select Save to Files:
If you have any problems following these steps, please get in touch.