Then PDF is probably not the format for you.
PDF is actually meant to behave as if it is going to be actually printed, even if you never do it. The dimensions of the document are physical, cm or inches or whatever.
You would need to match the document dimensions and internal resolution to the resolution of the monitor displaying it and the display mode of the document itself.
For example, you can put an image of 1920x1080px, in a document maintaining that 16:9 proportion, displayed at full screen, viewed on a FULLHD monitor. If one of those 4 conditions are not met, you will not see the actual pixels.
Just use a normal raster image. Either PNG or JPG will work best for you.
Edit 1. Ps will "resample" the content of the PDF. Inside a PDF you can potentially have different images with different resolutions, you can have a background at let's say 100PPI, and a photo on 300PPI, and some text that are resolution-independent, so PhotoShop needs to decide what resolution use to display the image,
It will probably assume that you want a typical high resolution, this is 300PPI, so even if the image you want to focus on has a lower original resolution this will be resampled, so, if the image was 100PPI and measuring 1000px now will be resampled to 3000px, unless you have the information about what was the initial resolution on the image inside the PDF, so you can assign it when importing the image into PhotoShop.
A keyword here is "Importing", you do not OPEN a PDF inside PhotoShop, you import it. The difference is that opening an image will preserve the important internal data, in this case, pixel size. Importing will adapt the information, but in many cases changing it.
I understand that you need to edit the PDF, but besides the resolution, you will lose the interactivity.
Now the answer is that Ps is not the program for you. It is better to use AcrobatPro to edit your PDF.
If you have the source material, you probably need to adapt the assets before exporting the interactive PDF.