The XSLT stylesheet [OXYGEN_DIR]/frameworks/dita/DITA-OT/plugins/org.dita.pdf2/xsl/fo/static-content.xsl contains templates which output the static header and footers for various parts of the PDF like the prolog, table of contents, front matter or body.
The templates for generating a footer for pages in the body are called insertBodyOddFooter or insertBodyEvenFooter.
These templates get the static content from resource files which depend on the language used for generating the PDF. The default resource file is [OXYGEN_DIR]/frameworks/dita/DITA-OT/plugins/org.dita.pdf2/cfg/common/vars/en.xml. These resource files contain variables like Body odd footer which can be set to specific user values.
Instead of modifying these resource files directly, they can be overwritten with modified versions of the resources in a PDF customization directory as explained in DITA to PDF Output Customization.