Hi Christian,
The way I would attempt to fix the size of any charts on a PDF export would be to use the layout container to either "Fix" the size or share the space as a percentage. For an A4 sized sheet you have 8.3 inches to play with so setting each one to 4 Inches should leave you space for a small header or you can play with the numbers as you wish. Looking at your image it looks like you have tried to set the Layout configuration but I think you need to click on each table listed below this and set the height per row configuration as each table is considered it's own row.

You can get to these controls by clicking on the layout tool and selecting each object listed in the per row configuration and setting the height.
If the size of the table goes over the page size then it may still fall onto a new page so you would need to adjust things like the font size.
Visibly you do not want to "squash" the table to the point it is unreadable so it may be a better experience for your consumers to have the charts in two pages and be able to read them. Otherwise if they should be a comparison made between these two sets of numbers combining them into one chart that clearly shows the relationship would be my preferred method.
Good luck and let us know if these settings helped.