well, we have the license in the institute where I work, so that is not a problem...

I must say that I seriously under-estimated the space required:

# du -h -d 1 /usr/local/Wolfram/Mathematica/11.1/Documentation/English/
221M    /usr/local/Wolfram/Mathematica/11.1/Documentation/English/Index
77M     /usr/local/Wolfram/Mathematica/11.1/Documentation/English/SpellIndex
4.4G    /usr/local/Wolfram/Mathematica/11.1/Documentation/English/System
76M     /usr/local/Wolfram/Mathematica/11.1/Documentation/English/Packages
4.7G    /usr/local/Wolfram/Mathematica/11.1/Documentation/English/

I still do not have any idea how to open it from my raspberry however: the index dir contains just these 3 files...

# ls /usr/local/Wolfram/Mathematica/11.1/Documentation/English/Index/
_d.cfs  segments.gen  segments_t

and the links between help pages do not work, reporting not found on links of this kind: paclet:ref/<function name>