Why can't I merge?

I have recently translated a PPT file where I couldn't merge the segments (option was greyed out), and assumed that this was because the segments were inside a text box or similar. Now I've moved onto a pdf file, and I've noticed the same thing.

This is what I am doing: ctrl click the left hand column of both segments to be joined, then right click. Whichever order I clock the segments, the optin is always greyed out. What am I doing wrong, or is this because the files are not pure text files?