Zotero uses the pdfinfo and pdftotext utilities from the open-source Xpdf project for PDF indexing.
If installation via the Search tab of the Zotero preferences window fails, the easiest solution is to install the tools from another computer running the same OS and copy the pdfinfo* and pdftotext* files over from the Zotero data directory on that computer. (On OS X and Linux, be sure to preserve the executable bit on the executables and script files.)
If you wish to build the Xpdf tools manually or use preinstalled system versions (e.g., from the Poppler fork), you must create a number of files in your Zotero data directory:
pdfinfo-{platform} and pdftotext-{platform}, where {platform} is “Win32”, “MacIntel”, “Linux-i686”, etc. To determine your current platform, open the Firefox Web Console and enter navigator.platform.replace(' ', '-'). The Windows executables require the .exe extension (e.g., pdftotext-Win32.exe).pdfinfo.vbs and hide.vbs should be saved as pdftotext.vbs.pdfinfo.sh, with the executable bit set.{executable-name}.version (e.g., pdftotext-MacIntel.version).