Display PDF calendar entries in modal

[atomtay]

Describe the feature. What should a user be able to do?
As a user, I should be able to preview non-digital (2019 and prior) calendar entries before downloading them. After previewing, I should be able to download the PDF or go to another piece of content.

Why do we need this feature?
It's harder to browse the older calendar entries when you need to download them first, then remember which file goes with which thumbnail, then open the correct one(s). The 2020 and beyond visualizations are much easier to browse since you navigate them as pages.

Alternatives
We could embed the PDF directly into a page.

Additional context
We used react modal successfully (and fairly easily) in Zoning Atlas and could use a similar implementation.