Report PDF layouts can be edited to look different than the normal on-screen report layouts.
For instance, an on-screen table layout can be set to show 25 rows, whereas the PDF table layout could instead be set to show 60 rows.
After clicking the red PDF export button:

...you'll be in the PDF print preview mode.
You can then select a report table, then: Ribbon > Table > Rows per page

Notice you can also change Print Current Page to the more extensive Print All Pages, which disrupts the original layout but ensures all table rows will be exported no matter what.