How to remove this adhesive, is it a hard job?
It’s funny we have had several questions with these tiles lately, the adhesive is asphalt based or it appears to be the same as I have seen many times and it will clean up with mineral spirits but takes a fair amount of work. 

Since those tiles are usually asbestos filled for the last 20 years I have recommend removing loose ones filling the void and overlaying. 

I have put down epoxy /pebble , carpet and regular tile and vinyl floor coverings over this type of tile, it looks like you are having better luck at removing the tiles so maybe that stuff is drying out, but use caution as the tiles did normally contain asbestos. 

So yes it is a lot of work to clean it up and leaving the tiles in place may be a better plan unless they are loose , normally Around the edge I find a few loose ones. 
I use self leveling compound to fill the void it has stayed in place well even where vinyl flooring was glued over the top and the epoxy pebble (yes it can be done indoors and looks much better than a heavily cracked 1920’s floor).