PDF export omits special characters like µ and Ω

Summary of Bug

When a page is exported as PDF, special characters like Ω and µ are omitted.

How to reproduce

1. Create a page, and include special characters like Ω
2. Save the page, and then 'Export as PDF'
3. In the PDF file, the 'Ω' character is completely missing

Findings

I have tried this in Confluence 4.0.3, 4.0, 3.5.9, 3.4.9, 3.3.3, 3.1.2 and 3.0.0_01. All of these versions suffer from the same problem. Have tried disabling and re-enabling the PDF Export plugin, then restart Confluence instance, but problem persists.

This is how it looks like in the editor :

This is what happens after it is exported as PDF :

Workaround

Upload a font that contains these symbols to Confluence, as detailed in CONF-25887.