I am using Apache FOP in XSL to display an XML file in pdf format.

The following xsl code displays the letter D in the pdf file when I am
expecting the delta (or increment) symbol 'Ä'
<xsl:text>&#x2206;</xsl:text>

Thiss is the "forward difference" character, which does
not seem to have a glyph in the standard fonts. It is
surprising though that you get a 'D', rather than a '#',
which FOP version are you using?

But the following xsl code displays the 'ÿ' character in the pdf file which
is correct.
<xsl:text>&#xFF;</xsl:text>

There is nothing surprising about this, as &#FF; is the
"lower latin letter y with diaresis", resulting in the
glyph you expected.

In order to get a "forward difference" character, get
a font which has a glyph for it, follow the instructions
to tell FOP about it, and select the font in the context
where you want to display this character. Alternatively,
if you don't care too much about possible fine differences,
you can try an upper greek letter delta, Δ