I can't say as I have a solution to this problem, but I have made it work for me. It seems the
renderWatermark() function at or around line 465 or pdfmake.js has problems with encoding the font.
var encoded = watermark.font.encode(watermark.text); returns a blank string, and doesnt have the extended properties the function is looking for later on.
pdfKitDoc.addContent('/' + encoded.fontId + ' ' + watermark.size.fontSize + ' Tf');
pdfKitDoc.addContent('/ ' + watermark.size.fontSize + ' Tf');
pdfKitDoc.addContent('<' + encoded.encodedText + '> Tj');
pdfKitDoc.addContent('(' + watermark.text + ') Tj');
I was able to get a watermark to show up in the correct position, albeit with a generic font rather than anything I had selected.