The extension Downloadable Files makes downloads list in the tab "Documentation" on a product page.
Besides, you can use external links in the filename field in the download form.

Features:
+ compatible with Opencart 3
+ support seopro
+ support Journal2 (mb it works with another templates)
+ support external links in the download form and in the downloads list
+ filename 128-character limit increased up to 255 (for another table prefix than 'oc_' you must edit install.sql)
+ files will be open in browser if its possible
+ files and links will be open in new window
+ support English, Russian and Ukrainian English only from 1.2.4 (see readme file if you want to add your language)

If you want to make a working with files and links esier and faster, you can use an extension Easy File Attacher.

Downloadable Files: troubleshooting.

Support
Email for bug reports and propositions: support@helpforsite.com