In LyX, how can I set Ubuntu's document viewer (Evince) as the preferred PDF reader for the generated PDF files?
All of a sudden it started using Okular, which I don't like all that much.
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 cannot explain why, but that worked for me:
In LyX 2.0.4 in Tools > Preferences must be something as
that you can change to other options, as evince:
In theory, that must be enough. This change must be saved in
However, I was surprised because in my Ubuntu this change does nothing, even after several reconfigure and restart (bug or feature?).
So I tried, maintaining the above
This worked for me: Tools>Preferences>File Handling>File Formats choose 'PDF (pdflatex)' in 'Format' and 'evince' in 'Editor' and Save