Is shopify compatible with FTP? Can I upload some custom HTML pages via FTP?
I have a site that was created using a database that creates product pages that are not indexable by Google. I wanted to create some duplicate html pages that are indexable by Google and upload them via FTP.
It is not compatible with FTP. You’d have to look at creating an app that would receive the FTP and process it into an API call into Shopify.
However, to your use specifically, Google has very stern warnings about creating duplicate pages. I’d say to just don’t do it at all 
Out of the box, Shopify theme pages are perfectly indexable by Google without any fuss.