I am generating a PDF from an XML file using XSLT, XSL-FO, Xalan and
FOP. My reader is Adobe Acrobat Reader.

For certain text, I am required to use Arial and Verdana fonts. I have
not been able to do this successfully. Times, Helvetica are rendering
fine, but not Arial and Verdana. I would appreciate your suggestions.

Times and Helvetica are predefined fonts in PDF and therefore available
out of the box. Arial and Verdana are not available out of the box,
you have to get some font files and use them according to the
instructions:
 http://xmlgraphics.apache.org/fop/0.92/fonts.html#custom