Despite its size and shabby appearance, The Pen deals out damage and knockdown on par with most other knives. The stabbing (or rather "poking") animation is absurdly fast, with the relatively low damage being a minor problem due to being able to strike several times where a knife would only manage a single swing. A downside is that The Pen's range is somewhat lower than most knives.
While this item is called "The Pen", its actual model is of a pencil. It's in-game and asset file names are a reference to the quote "The pen is mightier than the sword" by English author Edward Bulwer-Lytton.
Ironically, the actual sword is a great deal more powerful than the pen in PAYDAY 2.
The melee weapon is a reference to a quote from John Wick's origin film, wherein Viggo Tarasov describes a scene where Wick killed 3 people in a bar with a pencil. The teaser GIF on the Day 8 announcement further references this.
In retrospect, the reference to a Pen melee weapon was made in one of Wick's earliest quotes ("You have no idea the damage I can do with a pen...").
The Pen is the second melee weapon to be claimed with an activation key after the Alpha Mauler.