The article states that the undocumented pay less in taxes "partly because they earn less but also because many are paid off the books and escape payroll taxes." If they're paid off the books, it's safe to assume they receive little to no work benefits in addition to earning lower wages. The study also mentions that those who use falsified Social Security cards to gain employment are in fact paying money into the Social Security system that they will never reap.
In response to those who claim the undocumented pay no taxes and yet reap benefits paid for by tax revenues, in addition to those who do in fact pay income taxes:
"'There's sales tax, there's property tax, there's consumption taxes on alcohol, on cars, on gasoline, on utilities,' said Jeffrey S. Passel, a demographer at the Pew Hispanic Center who co-authored the report."
Keep in mind, however, that immigrants in the D.C. area tend to earn more money and are more likely to have documents than those in other parts of the country because of the presence of "think tanks, embassies and the high-tech industry". That means that D.C.-area immigrants probably pay more in taxes than those in other parts of the country.
The study comes from the nonpartisan Urban Institute.