LyX: change PDF viewer

Go to Tools > Preferences > File Handling > File Formats. Choose the file format. I am guessing that you want PDF (pdflatex). Change "viewer" to "evince". You are done.

The reason it all of a sudden started using Okular is because you installed Okular after you installed LyX and you did Tools > Reconfigure (or you did something that triggered that).

Why does LyX favor Okular over Evince. Here is my interpretation: Evince is the default viewer in Ubuntu. Suppose you install a program that is not the default program (e.g. Okular). If I had to make a guess as to whether you prefer Evince or Okular, I would guess that you prefer Okular because you went to the trouble to install it, not the program that came with Ubuntu.

I use a newer version of Lyx (version 2.1.4) and I can no longer follow the answers here. The problem is the same, which is that the default pdf viewer is changed to okular after I do the "reconfigure." What I did to solve the problem is the followings.

1. Go to /.lyx. There a lyxrc.defaults which contains many key-value pairs for Lyx configurations.
2. Make a backup with cp lyxrc.defaults lyxrc.defaults.bkp.
3. Edit lyxrc.defaults to replace all occurrences of okular with evince.