Earlier quoted context omitted.
> How will it effect? My guess is that the increase number of immigrants in Western Europe will cause a reduction in social benefits
How? Without immigrants, the social benefit system would reduce anyways because there aren't enough young people to pay into the system. In addition, both low and high skilled migrants increase productivity and per capita GDP in advanced economies (Source : IMF) [0]. [0] http://www.imf.org/external/pubs/ft/weo/2016/02/pdf/c4.pdf
Also I think that, whether low or high skilled, when you have high unemployment rate (above 10%), more workers just mean more people unemployed, so more social benefits, that the various welfare states won't be able to assume, since they all are in deficit.
Also can you point in the document where this affirmation is made?