We're in the process of updating the documentation for Zotero 5.0. Some documentation may be outdated in the meantime. Thanks for your understanding.
Zotero creates an index of the contents of your library, including the full text of attachments, in order to facilitate searches. The Search preference pane includes buttons to clear and rebuild the index. It is also possible to set the upper limit on indexed characters per document. The default is 500,000.
Zotero uses two plugins, pdftotext and pdfinfo, to index the contents of PDF files. The “Check for Update” button will ensure that you are using the latest versions. By default, Zotero will index up to 100 pages of any text-based PDF file, but this can be increased or decreased in this section.
See PDF Indexing for more information.
The Index Statistics section provides, as you might expect, details about the size of your index. Included are the total number of indexed, partially indexed and unindexed files, as well as a count of all the words in these files.