@Abhishek made a typo. The mv arguments should be in source, destination order:
find ~/Desktop/1-9-5 -name '*.pdf' -exec mv {} /home/najkarika/Desktop/345/ \;
The -exec option tells to the find command to execute another command on each matching file. \; marks the end of that external command. And {} is a placeholder for the name of the matching file currently examined by find.
EDIT: take care of not mistakenly use the backtick ` instead of a single quote ’
EDIT2: I took the liberty of changing the thread title. Don’t hesitate to change it back if you disagree.