PDF documents handled by the TCPDF PHP class embed a hidden watermark link (tcpdf.org or variants thereof) into the bottom left corner of generated documents by default.

In cases where TCPDF is handling certificate generation or even invoicing solutions — particularly in business and training-centric adoptions of Moodle deployment — these clickable watermark links contribute to needless confusion for the recipient of said documents. Potentials to redirect users to an unfamiliar website, fostering perceptions of potentially malicious interference with official documentation or otherwise raising the potential for violation of employer/client expectations with said documentation are all undesirable outcomes.  

Some explanation of the various observations with this issue alongside some ad-hoc fixes are listed at http://kreativschmiede.blogspot.com/2010/12/tcpdf-getting-rid-of-hidden-credit-link.html and https://github.com/tecnickcom/TCPDF/issues/292 

For the Custom Certificate plugin however, it would be a useful addition to Site Administration >> Custom Certificate settings to have a toggle or replacement option controlling the embedding of this watermark link.

STEPS TO REPLICATE: 

1. Navigate to a site with mod_customcert v3.10 (2020110900) installed and enabled
2. Go to any course, add a Custom Certificate activity. Leave at default settings
3. Click on 'View certificate' to inspect the document generated (it doesn't matter if the certificate is blank)
4. Observe that when document is previewed or downloaded and viewed within a web browser, mousing over the bottom left of the document will reveal an invisible and clickable link to tcpdf.org