Your new iPad will be using a recent version of iPadOS - which may differ slightly from older versions in how email attachments are saved to the various Apps installed on your iPad.
Opening an attachment from the Mail App will not necessarily save the attachment/document - but will often open an attachment in Quickview - allowing review before saving the file to a specific location or App.
To save an PDF attachment to your Books App, touch-and-hold the PDF attachment in Mail; this will open both a thumbnail review and an Action Menu. From the Action Menu, select Share - which will open the system Share Sheet. From the Share Sheet, select the Books App.
If instead you wish to save the PDF attachment directly to your iPad's file system, from the initial Action Menu you can select Save to Files - or select this same option from the Share Sheet.