When you open a PDF web link under mobile Safari, the PDF document will be displayed for browsing from within Safari. It might be useful to save the PDF into iBooks for read-later such as when there is lesser Internet connection or to take advantage of the better PDF reader in iBooks.
When the PDF document is fully loaded, tap anywhere on the document to bring out the gray color toolbar bar. On the right end of the toolbar there will be buttons for “Open in iBooks” or “Open In…”.
Tap “Open in iBooks” to save the PDF into iBooks. Safari will save a copy of the PDF into iBooks and switch over to iBooks app. If the PDF size is big, it might take a moment for Safari to switch to iBooks. Tap on the “Open In…” button to choose a different app to manage the PDF document.