You can easily see the document ID of a particular letter in the address bar of your browser or at the bottom right of the document preview once you have opened the corresponding document.
It is the 25-digit-combination of numbers and letters that follows /app/document/.
Alternatively, after downloading the letter from your inbox, the document ID can also be found in the name of the PDF file in the last position (How are PDF files named when downloaded?).
The document ID is needed for almost all requests around originals & documents, so keep it handy when you have a request for a document.