Hi,
when exporting reports as PDF they usually, at least metric-based countries, use DIN-format which has an approximate ration of 1:1.41. The 4:3 or 16:9 are based on screen ratios. When exporting PDF I’d like to suggest to use DIN-based aspect ratio and factoring in the orientation portrait or landscape.
Adding to this, might be be useful to define a Print-CSS as the PDF generator uses Chromium? A Print-Preview might also be a nice addition 
PS: Just for completeness, letter based aspect ratios for US might also be a good option.
Best
Mike