As you are attempting to view a page that is formatted as a regular portrait page, when displayed in Landscape your iPad will natively show the full page-width. As such, you'll not see the full page-length.
Instead of using the iPad's native PDF viewer, consider using a third-party PDF App - of which there are many in the Apple App Store. In so doing, you may have more flexibility in page views.
You need to understand that viewing a full portrait page with your iPad in landscape orientation - respecting the aspect-ratio of the document page - will unavoidably result in large blank areas on either side of the page. As such, your music score may become too small to comfortably read.
Instead of using a PDF reader, have you thought about using a dedicated Sheet Music App - such as forScore?
https://apps.apple.com/app/forscore/id363738376