The page size (16.81 x 20.14 in.) is exactly the number of pixels at 72 ppi. There's nothing abnormal there.
The question is: What page size do you expect?
Your PNG are low resolution and Adobe Acrobat simply take what you have and doesn't modify them. If you expect a different page size or would prefer a higher resolution and smaller size, you'll probably need to resave your PNG with a higher resolution or maybe have a look at the preflight functionality of Acrobat Pro to modify the resolution quickly within the PDF.
To come back on your issue of Acrobat lowering the quality of your PNG: One thing you might want to verify is the resolution of your Acrobat preferences. If you use the default setting, it might show your image bigger when viewing at 100%. If you use 72ppi then it should show you the images as they truly are.
It's possible this is what makes the images look lower resolution. So it's worth trying a different setting that fits your screen.
You can also have a look at the PNG to PDF conversion preferences; by default they seem to be at medium quality. Maybe that's what you're referring to in your question and you already verified this.
If you aren't sure if your Acrobat is doing a good job, simply open a few of these PDF in Photoshop at their native resolution. If the image is the same and you're happy with the results, then the issue was probably only the resolution preview in Acrobat.
Acrobat doesn't always show images, texts and vector in a perfect way, and there's a few settings in your preferences that don't affect the file but still display them differently than in a software like Photoshop.