Hello,
You should be able to use docInfo.getSDFObj() to obtain the SDF/Cos low-level info object. From there you can get the value of your custom property.
Here is some PDFNetJS full code.
var docInfoObj = yield docInfo.getSDFObj();
var customPropObj = yield docInfoObj.findObj("myCustomProperty");
// Note the next statement assumes customPropObj is a String Obj.
// Use other functions such as getBool, getNumber, getName etc. to get the value
// of other object types
var customPropString = yield docInfoObj.getAsPDFText();
PDFNet on your server can also access the custom property in a similar manner.
David Tippett
Senior Software Developer
PDFTron Systems Inc.