XPDF is a PDF viewer for Linux. It can generate a fulltext version of a PDF and thus allow Nectil to index a PDF file for further user searching. It must be installed separately (it's not packaged with Nectil) and be available in the system path. No additional configuration is necessary.

Actually it's a binary (pdftotext) classicaly provided with Xpdf which is used by Sushee. On some distributions Xpdf is considered obsolete, but this binary can still be installed separately.
On Ubutuntu the package is now called poppler-utils.