When “Download attachment PDF with watermark“ is enabled (by default), downloading PDF attachments will be watermarked.
If "Export page and download PDF attachments with extra watermark" is turned on, the PDF attachments watermark will follow the additional settings; if it is turned off, it is consistent with the page watermark. ( 2.2.2.8 Export page and download PDF attachments with extra watermark )
Example:
Downloaded PDF attachment
...