Zotero requires a customized version of pdfinfo that can save its output
to a text file, so it can't just check for an existing version. You can
create a symlink to an existing version of pdftotext on Linux and OS X
(but not Windows, since in addition to the pdfinfo patch Windows
requires modified binaries that run in the background).
See http://www.zotero.org/documentation/pdf_fulltext_indexing for more info.