Hey @Colour Me Tender 

So everything is good for profiles to receive the email with the download link in the welcome flow for new subscribers.

but for existing subscribers

Create a segment that includes all existing subscribers. You can do this by defining the segment with a filter like "Subscribed to List at least once atleast 30 days ago." 

Now I would NOT suggest you create a flow as profiles in a segment/List can only go through the flow once. so creating a flow would not serve your purpose of sending them the link to download every month.

So instead i would suggest sending a campaign every month to the above created segment and give the link to download in the campaign. 

Let me know if you have any questions and thank you for being part of this amazing community

Cheers

Arpit