That is, because it is not possible unfortunately. There are options to search attachment names, titles, descriptions etc.., but it's made for regular attachments only – no file content search.
I'm working actively on an algorithm to allow searching attachment contents, but currently there is no reliable solution to read different PDF, doc, txt or other file contents for indexing within PHP. There are just too many file type variations, encodings and other issues to fix. Sorry about the bad news, but there is honestly nothing I can do about it right now.
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