@Haikosli Your best option is using a digital downloads app from Shopify App Store. Shopify has its own free app, and there are plenty of freemium options too like ours - Filemonk - that has more features and updates. 
Here’s what you’ll need to do for any of the apps, including ours. It takes just a couple of minutes -
- Create your product listings.
- Open the digital downloads app and upload your files to it
- Link the files to each product listing
- Configure emails or add a “Download” button on Thank you page.
After you set this up, Filemonk will process all orders that are placed on your store. If it finds any products that have files linked to it, your customer will get an email with a link to download their files and they will also see a “Download” button on Thank you page.
Let me know if you have any questions!