I had ipad 8 and iphone 1a pro, both registered with the same account and updated to the latest version. When i use iTunes to add pdf file from pc to my iPad books app, it didn’t showed on my iphone books ! Any help Thanks in advance.

Make sure that "iCloud Drive" is activated in Settings > Books. Also check that "Books" is activated in Settings > [your name] > iCloud. You may want to do this on both the iPad and the iPhone.

Thanks for you reply; I think I understand the issue better now. I haven't used iTunes for the longest time since I'm using a Mac. Nevertheless, I remember that there was a way to select which books/PDFs were synched between iTunes and your device. From what you are stating, I gather that the transfer works fine between iTunes and your iPad. What happens when you connect your iPhone to iTunes? Does the PDF make it to your iPhone? I understand that this solution may not be the one you were seeking but I cannot think of anything else.

I hope another contributor could chime in and offer some advice.