ActiveCampaign Subscription add-on for WordPress Download Manager Pro gives you an option to add download manager subscribers to your ActiveCampaign list. You simply need to enable an email lock for a package. So when user will enter their email address to download the package the email automatically will be added to your selected ActiveCampaign list.

You can choose a different list for each package/download or use a global list for all of your downloads.

Setting up Active Campaign Subscription:

Install and activate the add-on first.
Go to Dashboard → Downloads → Settings → Active Campaign. Save API URL, API Key and global list. Get your API key, URL from your Active Campaign account.
When adding a new package/download activate Email Lock Package settings. You can set a unique list for the current package. You can also use the global list.
Now the user must submit their email address to access the download. And submitted email will be added to your contact list in Active Campaign.