Steps to Reproduce:

1. Create a Page in Confluence.
2. Add some text in Korean Language.
3. Add a code block macro into that page.
4. Add some Korean text into Code Block Macro
   Make sure that you have installed Korean Font in General Configuration > PDF Export Language Support.
5. Export that Page to PDF

Expected Result:

Exported PDF should show texts of code block macro.

Actual Result:

Exported PDF doesn't show texts in code block macro.

Workaround

Add the below line to the PDF stylesheet to specify the font that supports Korean

.code pre, .preformatted pre {
    font-family:ConfluenceInstalledFont, Arial;
}