Unredact Threatening Document
The “Unredact Threatening Document” is objective no. 2 in the holiday hack challenge 2019 season.
To solve this objective, enter into the Quad for searching a threatening letter. You will find a threatening letter in the top left corner.

After clicking on the letter, a PDF file will open. The PDF is a letter written by a concerned and aggrieved character to the administration, faculty and staff of ELF university.

There is a hidden subject line, which is not legible. Now, our task is to read the text in the subject line. Just do the following to unmask it.
Right click -> Inspect Element

Also, you can press F12 to open the developers’ tool. Further, inspect the element using the inspection tool.

Finally, When you go through the span element, you will find that the first letter of the subject is “DEMAND”. This is the answer.
VPN Hacks has explained only one way to solve this objective. You can answer using other methods also.
Earlier Objectives
Before this we have two objectives that is “Talk to Santa in the Quad” and “Find the Turtle Doves“.
Santa is standing in the Quad. He is holding an Umbrella. When you click on his name, he talks to you and guides what to do next. To reach Santa, keep on clicking towards the direction he is standing.
When you visit Santa for the first time, he will ask you to find turtle doves. Also, he suggests you view objectives in your badge. Your badge is on your chest.
To find the turtle doves, you hope around the campus of ELF university. In the Student Union near the flame, you will find turtle doves. When you click on turtle doves, the next hint will open.

The next objective is “Evaluate Attack Outcome” by doing windows logs analysis.