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1. The bonus of a subdomain is its still sort of linked to your site and can be easier to treat separate (organize) but the con is it doesn't really help boost your main domain 2. having it on your main domain e.g. /docs can help boost your main site (if people link to PDFs etc.) but can obviously increase site size (and sometimes load speeds) and can get a bit messy with orgnaising. End of the day it comes down to your preference and whats easier for the user there are pros & cons for both personally I tend to lean toward having it on your main domain just to increase its authority and value to users (and thus Google). Good luck.