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Workarounds are available and they are even advantageous in some ways:

Provide links to the Knowledge Articles in your public knowledge base (PKB), portal or community. "Give someone a fish and you feed them for a day. Teach someone to fish and you feed them for a lifetime." See for Including Knowledge Article Links to your Public Knowledge Base (PKB) in Emails for more information.
Our sample Visualforce email template (for advanced administrators and developers) allows you to precisely format and style PDF output, but development is required. This is not supported by Internet Creations, but we have provide a basic guide and sample code to get you started. Visit: https://ic.my.salesforce.com/sfc/p/300000000T9Bp2AT_AXcYjXWNI8Pqd.ovH2Mwng=

This is a known limitation in Email to Case Premium due to a limitation in the Apex Single Email method.

http://boards.developerforce.com/t5/General-Development/Attaching-Knowledge-Articles-as-PDFs-in-Apex-driven-emails/td-p/208800

http://kb.internetcreations.com/?kb=000004410

Internet Creations