Which barcode are you using? I was able to leave the leading zeroes in an EAN8 barcode just by supplying the rawvalue as a text string.
this.rawValue = "00001234";
I was using the calculate event for that previous example, but the PDF417 BarCode doesn't allow that to be modified. So, I would suggest a button or other valid event.
A quick test to verify it's working.
if (PDF417BarCode1.rawValue == "0000001234") PDF417BarCode1.rawValue = "9876543210";
else PDF417BarCode1.rawValue = "0000001234";