As DominoPDF converts the HTML representation of a form / document to PDF you have a number of options to meet your requirements.
1. Set the background colour of the Form to the "parchment" colour you desire.
2. Set the background image of the Form to the "parchment" image you desire. Note, that in earlier versions of Domino the "tiling" options were not supported for HTML output so ensure your document appears correct in a browser.
3. Add HTML code to the BODY tag of the form to add your image background. For example; "BACKGROUND=\"parchment.gif\"" (note the escape slashes that @Formula language needs.
What we're trying to achieve is to tell DominoPDF that the HTML used for PDF conversion will have some form of background effect, either colour or an image. DominoPDF will then render this appropriately when output to PDF.
Option 3 is prefered as it gives better control over the output but all should work accordingly.