This is most probably because you have attempted to associate the AxCrypt file extension (.axx) with a Adobe Acrobat PDF viewer.
When you install AxCrypt, the .axx file extension is associated with AxCrypt, so when you double-click such a file, AxCrypt is started (which may in turn start another application such as a PDF viewer after decryption).
If you associate .axx directly with something else than AxCrypt, it will not work.
If this does not explain it, please add a screen shot of the situation. You might also want to google “windows file association extension” or something similar.