After plotting, publishing, or exporting a drawing to PDF from AutoCAD 2016 and newer, when looking at the PDF file in a PDF viewer, each block of SHX text has been put into the Comments List in the PDF.
This feature was introduced in AutoCAD 2016 to allow for searchable text in a PDF when SHX fonts were used in the originating DWG file. It does not affect TrueType fonts.
On the command line in AutoCAD, enter EPDFSHX and change the value to 0 (zero). In AutoCAD 2017, the command PDFSHX maps to EPDFSHX (either may be typed in). For AutoCAD 2016, the core AutoCAD 2016 Service Pack 1 must be installed in order to have the EPDFSHX variable (see Autodesk Downloads).
To workaround not being able to change EPDFSHX, use AutoCAD 2015 or older to create PDFs, use a third-party PDF printer driver (in AutoCAD go to File > Print > and choose Expert PDF as printer then create a PDF document), or change the text in your drawing to a TrueType font.