I am using Xalan to convert an XML file to pdf rendered for display

Xalan is an XSLT processor, it doesn't produce PDF (well, usually).
Pleas tell us what FO processor you are using.

using Adobe Acrobat.

Do you mean Acrobat *Reader*? If so, it is known to produce
visibly different line widths for borders specified to have
the same width on computer monitors. A printout should
look ok though.