Document -> Settings -> Fonts -> Roman -> Latin Modern Roman
How to get Clear Fonts in lyx output PDF
For those who write their thesis papers, research papers or any article, Lyx is one of the most user friendly and trouble free Latex processor that can be used with latex. Usage is as easy as using any document processor like Microsoft word but lets you harness the power of Latex at the same time. But I have had a problem when generating the pdf using pdf2latex or any other tool that the font of the output pdf is somewhat unclear and blurred. To solve this do the following setting.