I would link to both the website and the pdf (and say which link is which)
This helps with a few things, such as the possibility of a browser/computer/location blocking one link and not the other. And it also lets others see not only that the pdf exists but also where exactly it came from. Unfortunately this does mean you usually wouldn't put the link on the actual text or name of the UA article since you'd need to specify whether it's the website or the pdf.
The Bard's Magical Inspiration feature states:
If a creature has a Bardic Inspiration die from you and casts a spell, the creature can roll that die and add the number rolled to one damage or healing roll of the spell. The Bardic Inspiration die is then lost.
- Class Feature Variants Unearthed Arcana (page 3) [link to site] [link to pdf]
The Class Feature Variants Unearthed Arcana includes the Magical Inspiration feature for Bards which states (link to site, link to pdf):
If a creature has a Bardic Inspiration die from you and casts a spell, the creature can roll that die and add the number rolled to one damage or healing roll of the spell. The Bardic Inspiration die is then lost.
You could also do something like this, though it's slightly less clear/explicit I do think this is the best method, and is the one I'll likely be using:
The Class Feature Variants Unearthed Arcana states: (pdf direct link)
If a creature has a Bardic Inspiration die from you and casts a spell, the creature can roll that die and add the number rolled to one damage or healing roll of the spell. The Bardic Inspiration die is then lost.