Relaying my findings from the more recent question: visualforce page as PDF - unicode font bold characters
According to Flying Saucer PDF Generator and Unicode you need a special variant build of Flying Saucer to:
generate the appropriate PDF commands to emulate bold and italics when a style calls for it but no matching font can be found.
From the Features of this Release:
Added font emulation for bold and italics variations when there is no direct support in the font files themselves. Fonts like Microsoft's Arial Unicode MS only come in one version: plain text. In order to have bold, italics and bold+italics the font must be modified on-the-fly by the PDF display software.
So, unless Salesforce were using a variation of Flying Saucer or switch to another PDF renderer it won't be possible.
As per the comment by Oleksiy, voting for the idea Arial Unicode MS - Bold in PDF may help in the long run.