Exporting to PDF from InDesign always preserves the document dimensions, so about the only thing I can think of is the page size was changed using the Page tool (which would not show up under Document setup), or you are printing to postscript and distilling instead of exporting.
Is the change evident when printing? Acrobat defaults to "Fit" in the Print dialog box, so the dimensions inevitably change. Change scaling to "actual size". (I'm on my phone and the exact working is eluding me, so it might be "do not scale"? I'll check when I get to back to a computer.)
If the page dimensions are changing in the file and not just via printing, you can tell by positioning the mouse in the lower left corner of a page, just to the right of the Navigation bar. The page dimensions will appear and disappear as you mouse over the area.
Thanks for the suggestions. I went back through my process and found that the PDF was the correct size. However, Photoshop didn't import it correctly. I tried placing the PDF in Illustrator instead and it worked just fine without distorting it.