I am encountering the same issue. No text is extracted. Have you solved yours?
New installation. RS 9.0 running on Windows server 2016 IIS.
This field is untounced in config.default.php
$extracted_text_field=72;
In config.php
$pdftotext_path='E:\pdftotext';
PDF previews are created successfully. Not sure if this info is related - unoconv is used and Office document previews are created successfully.
Debug log is enabled, but I didn't see any calling of pdftotext.exe when the pdf files are uploaded and previews are created.