I recently installed lyx on a new computer. And it seems that I can't preview the pdf in adobe reader or sumatra. The yap software is invoked to preview the pdf. I tried to add the adobe path, change the preview to AcroRD32 or Summatra. I even re-installed lyx. Nothing works so far. This is really bothering me since I need to adjust the look of the paper. With yap, I can't really see how the paper looks like now.
I only have lyx 2.0.8 in Linux, without Sumatra nor Acroread, but I hope this guide help:
Tools > Preferences ... >
Ouput > General > Foward search >
PDF command >
SumatraPDF -reuse-instance $$o -forward-search $$t $$n
File handling > File formats >
PDF (pdflatex)(for example)