Adobe Acrobat 5.0 is slow — why?

I am supporting someone with 128 MB of RAM, plenty of hard drive space on Windows 98 running Acrobat 5.0. Every time she tries to open a .pdf file her system hangs–especially when she is trying to work in other applications.
We have cleaned up hertemp files and defragmented her disk drive more than once.
What could be causing this Acrobat slowness?