Image quality severely degrades when trying to create a PDF

Hi - I am trying to create a PDF of a series of PNG images in Adobe Acrobat. The PNGs are high-resolution, but when I import them into Acrobat, the quality is severely degraded.

Under the "Create PDF from Multiple Files tool" I selected the "Larger File Size" option, which, according to the pop up window, would retain the image quality...but that was not the case.

I imagine there's something I'm missing. Can anyone help?

Thanks in advance