Image rotation between Illustrator and Acrobat?

I have several dozen one-page PDF files (slides with images and text)
which I want to use in Adobe Acrobat or Reader, after I've edited and
touched them up in minor ways using Illustrator.

The slides look like normal landscape mode slides when opened in
Acrobat, but when I open any of them in Illustrator CS the entire
graphic is rotated by 90 deg CW (it's like they're in landscape mode,
with the paper rotated 90 degrees). I can still edit the content in
this orientation -- I just get a crick in the neck doing it. If I Save
them after editing in Illustrator they're back in proper landscape mode
viewed in Acrobat.

Where or how is this difference in image rotation -- which is apparently
built into the original PDF files -- being set or communicated between
the two programs? Could I somehow "turn off" the rotation information
in Illustrator? Or somehow remove it from the original PDF files?

Thanks for any enlightenment.