What was the physical image size in Photoshop? For instance, if it was set to something like 6 inches x 4 inches at a resolution of 600dpi, then the PDF preset would down sample contained images.
Generally speaking, always save Photoshop images using one of the High Quality print presets, and definitely don't flatten before doing so. This is especially important if you have any Type layers.
We will need to know the exact PDF settings used.
Here is every detail, I hope.
Photoshop Image Size
Width = 2550px
Height = 3300px
Width = 8.5 inches
Height = 11 inches
Resolution = 300 pixels/inch
SAVE AS PHOTOSHOP PDF
1. Uncheck LAYERS
Adobe Preset - I've tried High Quality Print and Press Quality
Standard - None
Compatibility - Acrobat 5 (pdf 1.4), BUT I've tried all the options
Options - Uncheck Preserve Photoshop Editing, and Embed Page Thumbnail, Optimize for Fast Web Viewing... although I've tried variations of check and unchecked.
Options - Do Not Downsample, but I have tried the other options
Compression - JPEG
Image Quality - I always use Maximum and High
Color - Color Conversion - No Conversion
Nothing changed on SECURITY.
I have plenty of Photoshop documents, where Ive "saved as" a .pdf and the pixel dimensions DID NOT CHANGE, so I know my issue is not the norm. I've tried to check all possible options within Photoshop and can not figure what is dropping it from 300 ppi to 72ppi
As I'm writing this I did a test document. The psd is 65.74MB 2550 x 3300, 8.5 x 11 at 300ppi
As viewed in Bridge, the .pdf turned out to be 10.11MB, 612 x 792 ... doesnt show ppi But 612 x 792 is what the pixels would be at 72ppi.
I hope this explains better. Help.