Have already upload a PDF file to Salesforce, added a link in the custom page, click on the link to download the PDF file, at the same time the login name in the form of watermark is added to the PDF, my problem is I don't know how to add watermark into PDF

Have already upload a PDF file to Salesforce, added a link in the custom page, click on the link to download the PDF file, at the same time the login name in the form of watermark is added to the PDF, my problem is I don't know how to add watermark into PDF

Have already upload a PDF file to Salesforce, added a link in the custom page, click on the link to download the PDF file, at the same time the login name in the form of watermark is added to the PDF, my problem is I don't know how to add watermark into PDF

Have already upload a PDF file to Salesforce, added a link in the custom page, click on the link to download the PDF file, at the same time the login name in the form of watermark is added to the PDF, my problem is I don't know how to add watermark into PDF

Have already upload a PDF file to Salesforce, added a link in the custom page, click on the link to download the PDF file, at the same time the login name in the form of watermark is added to the PDF, my problem is I don't know how to add watermark into PDF