You can find downloaded PDFs of book which you downloaded on your mobile phone in the Files app or a similar file manager app.check your phones files whenever you download some PDFs or anything they get saved in your phone so it is .ost likely that it may saved in file manager.

If you are unable to access the file use a PDF reader app to open and read PDFs on your phone. For example, you can download and install the Adobe Acrobat Reader app from the Google Play Store.