Can I download an iPad Pro 15 user manual as a pdf?

Currently no. Apple does not offer a way to download the entire manual as a PDF since its a very long manual and would be an exceedingly large file to download.

You can however navigate to "iPad User Guide - Apple Support" and use the Print as PDF option in your browser to save specific entries from the manual as PDF.

On an iPad:

Link-> Save a full page screenshot as a PDF on iPad - Apple Support

The iPad User Guide is a rich source of frequently overlooked information about your iPad and iPadOS. While the Guide has not been published as a PDF for many years, it is available both as an online version (accessed using a web browser of your choice - such as Safari), or as an Apple Books download:

iPad User Guide - Apple Support

https://books.apple.com/book/id1567104892

The Tips App, installed by default on all new models of iPad also leads you to the User Guide. In case you missed it, here is a link to the Tips App:

https://apps.apple.com/gb/app/tips/id1069509450

You might also be interested to learn that Apple also has a huge YouTube Channel - containing videos and feature guidance:

https://www.youtube.com/user/Apple

https://books.apple.com/book/id1567104892

You can download it in ibook