I don't understand, there are demos in the source code that show how to use poppler
poppler-0.78.0.tar.xz\poppler-0.78.0\glib\demo
poppler-0.78.0.tar.xz\poppler-0.78.0\qt5\demos
also here:
https://freedesktop.org/wiki/Software/poppler/
there is a part that says
"Non exhaustive list of programs using poppler"
check one of them to see how they applied poppler in their projects
if you can't port the code from cpp to delphi then try looking at these:
https://torry.net/pages.php?id=521
or find TMS components that deal with pdfs since the sources are written in Delphi