Once a letter leaves your office it travels as a PDF, gets printed and photocopied. The other side wants to know that the copy in their hands really came from you.
In our portal every document carries a 12-character verification code at the bottom right. A QR code can be printed as well, if you want one. Whoever types the code or scans the QR sees the status of the document on a public verification page.
The status is one of three: the document is valid, the document has been cancelled, or it has been replaced by a new one. You can cancel a faulty letter or write a replacement; the old copy no longer shows as valid when verified.
If you have the PDF, you can pick the file on the verification page and compare it. The fingerprint of the file is calculated in your browser and the file never leaves your device. The result tells you whether the file is identical to the document that was issued.