[PDFViewer] Pager not in sync when scrolling a document with different paper orientation and becomes empty when last page is reached

[mbechev]

Describe the bug
The built-in pager cannot properly calculate and scroll to the correct page when the uploaded PDF file has pages with different orientations (landscape and portrait). The pager correctly determines the pages until the page with changed orientation is reached.

In addition, when the incorrect pager calculation indicates that the last page of the document is reached, the pager becomes empty.

To Reproduce
Upload the following file:
paging-issue.pdf

and start scrolling the PDFViewer pages. See that after page 7 the pager starts to incorrectly count the next/previous pages and after page 25 (which is incorrectly calculated to be the last page), the pager becomes empty.
https://stackblitz.com/edit/angular-jgecpe?file=src%2Fapp%2Fapp.component.ts

The issue with the empty pager can also be reproduced when uploading the following file and scrolling to the end of the document:
Sample-test-file.pdf