PDF Slideshow photo orientation

I am a volunteer with a local Heritage group (Keith &
District Heritage Group

KDHG Scottish Charity Number SCO. 33492) and we often create
slideshows of old photographs. We find that the best way to do that is to
create the slideshow in PDF. We are a charity and find it difficult to justify
the cost of Acrobat so we use the free PDFcreator or PDFill printer software.
However, if we have a mixture of portrait and landscape photographs these
packages always rotate the portrait ones so that they fill the screen frame.
The only workaround we’ve found is to edit the portrait photographs so that
their canvas is adjusted so that they appear to be landscape. This is a real
pain but we have been completely unable to find why the limitation exists in
these applications. Can anyone tell me why the limitation exists, whether it also
exists in Acrobat and whether there is any workaround.