After exporting. The images of the same PDF are at different DPI

I noticed that when exporting to PDF, setting DPI is different.

While in JPG you set DPI here:

For PDF it is here:

I made tests, I noticed that in the PDF document the DPI does not necessarily stay the same in the whole document.

In PDF you can have an image at 200 DPI, while another image in the same document is at 72 DPI.

Quite different from JPG, where everything is at the same DPI.

Regardless of whether or not the images were at different DPI while editing.

Here I have a video demonstration, I imported a PDF in Corel.
And the images in the PDF are at a different DPI.

https://youtu.be/4TsGBLSyWCE

My question is:

Even if the DPI of the images are different during editing, how can I export to PDF and set everything at 300 DPI ?