You appear to be viewing a PDF using the native Files App - however, the illustrated thumbnail bar at the left of the screen is not the Files App sidebar.
The Files sidebar itself can be hidden:

By contrast, the thumbnail bar that you see in the Files App PDF viewer cannot be suppressed. If you find the thumbnail bar distracting, you might consider using a different PDF App - such as Adobe Acrobat Reader:
https://apps.apple.com/gb/app/adobe-acrobat-reader-edit-pdf/id469337564
Apple do invite submission of constructive comments and feature requests via their Product Feedback portal. For iPad and iPadOS:
Feedback - iPad - Apple
It is unlikely that your request will be the first. It is perhaps worth noting that the volume of requests received for a particular feature is likely a contributing factor towards the likelihood of the feature or request being considered by the Developers in assigning resources. As such, your submission might be the one that makes the difference…