I printed to PDF this HN thread in Chrome (I assume that the PDF printing was done on the system level by OSX -- EDIT yes, from the file: "/Producer (macOS Version 10.15.2 \(Build 19C57\) Quartz PDFContext)"), and none of the page's strings appear as ascii or utf-8 in the document. grep is unable to find any string in that file.
Do you have a specific print to PDF setup? Or a PDF-aware grep..?
EDIT: Seeing the command-line you're using, the search you do is over the files' names, correct? The PDF/(original web page) text content is not indexed, right? Just to make sure I understand correctly.