How can I open a downloaded PDF file on my iPad?

Your iPad can view a PDF file from the Files App. Simply open the Files App - and navigate to the target file.

If your PDF was downloaded using the Safari browser App, your file is likely to be located with your Downloads folder; unless you have changed your settings, by default the Downloads folder should be visible in the Files sidebar under Favourites.

Should you choose to do so, you can install a more sophisticated PDF reader App of your choice from the Apple App Store. A popular and well known App is Adobe Acrobat Reader:

https://apps.apple.com/app/adobe-acrobat-reader-edit-pdf/id469337564

You can install various apps from the App Store that allow you to open PDF files, or you can use your default web browser (e.g: Safari) to open them