In-Design to PDF Resolution Question

Your Photoshop file was flattened and converted to a JPEG when it was exported to PDF in the InDesign file. If the image's effective reolution was higher than the output resolution it was downsampled. If the image was cropped then the hidden part of the image was excluded from the export.

Photoshop files contain layers and masks, some of which might be hidden in the Layers panel or obscured by higher layers in the image. When the PDF was created the Photoshop file was flattened, turning it into one flat image. If you think of a Photoshop image as a background along with adjustments, masks, and other layers above it, then flattening makes the file smaller by about the about of space everything except the background takes up.

When you exported the image was compressed as a JPEG, which is a much more efficient compression format than Photoshop uses. JPEG is lossy, in that some of the image data os altered as a result of comression and decompression. For high quality images the amound saved is still a lot, yet the image is altered so slightly nobody would notice.

If the image is cropped then all the cropped out part of the image is not included in hte export.

If the image is downsampled it is saved in a lower resolution that the original imag.e but still enough resolution for printing or display.

All of these together will combine to make a large PSD placed into InDesign save as a much smaller PDF.