PdfHandler currently performs the ImageMagick call [on its own](https://github.com/wikimedia/mediawiki-extensions-PdfHandler/blob/1.32.1/includes/PdfHandler.php#L205-L215) using a very simple command line. As a result, for thumbnails of documents like [this one](https://upload.wikimedia.org/wikipedia/commons/thumb/4/46/PRcoords_Cheatsheet.pdf/page1-440px-PRcoords_Cheatsheet.pdf.jpg), the output is blurrier than ideal for textual pdfs.
A nice start would be taking what T8193 does ([code here](https://github.com/wikimedia/mediawiki/blob/1.35.0-rc.2/includes/media/BitmapHandler.php#L166-L187)) and adding a `-sharpen` when JPEG is requested.
On the other hand, since PDF is a document format with real vectors, there is an argument for using PNG in the output. The file size can be reduced by using a 8-bit PNG with `PNG8:` in ImageMagick. (Their handling of transparency [is not good](https://stackoverflow.com/a/14032098), but documents don't have that issue!) If the size is reduced enough, I would love to see them deployed on WMF sites.