How to copy pdf file to Books using File manager?

Go to the PDF file of interest on your iPhone or iPad and tap on it to open it.. Go to the share icon at the top and click it. A pane opens and on the 2nd row of options, you'll see options like AirDrop, Messages, Mail, and others. Slide this row to the left and at the end of that row you'll see an option for "More". Click on that option. This brings up a new menu and you should scroll down this menu to the option "Open in Book". Click on this option. Your PDF will be displayed using Books. Tap on the page that is displayed on your screen and you'll get another share icon at the top of the display. Click on this share icon and on the panel that opens is an option to Copy to Books. Click this. Now a copy of the PDF is saved in Books.