One Competitor of Nitro is Nemetschek Group, who owns Bluebeam Software.  Bluebeam is the industry standard in the construction industry for marking up pdf documents.   

Bluebeam has far greater functionality than Nitro software, and is the market leader in the construction industry.   If Nitro was to turn their niche into a profitalbe / attractive one, Nemetschek Goup could develop a cheaper, dumbed down version of their software to compete in this space. 

Nemtschek Group's Build Group (Bluebeam Group) grew revenue 25.6% last year to $177.7 M EURO, with EBITDA growing 47% to $$61.5 M EURO.  

Nemtschek Group's EV/S is around 13 (with 20% growth rate, +20% profit margin) vs Nitro at 9 (with 10% growth rate, -20% cashflow).