Find all files in the Documents directory with a .txt extension, and for each of those files, execute a command to replace all numbers with the same number followed by an 'a' (e.g. 1 becomes 1a). Create a backup of each file with the .bu extension.
find ~/Documents -type f -name '*.txt' -exec sed -i.bu 's/[0-9]/&a/g' {} \;