You can also use bash' command substitution operator (backticks) as
mv `ls -t ~/Downloads | head -1` ~/Documents
as a one-shot solution if you do not want to move multiple files in one go. See the bash man-page:
Command substitution allows the output of a command to replace the command name. There
are two forms:
Bash performs the expansion by executing command and replacing the command substitution
with the standard output of the command, with any trailing newlines deleted. Embedded
newlines are not deleted, but they may be removed during word splitting. The command sub‐
stitution $(cat file) can be replaced by the equivalent but faster $(< file).
When the old-style backquote form of substitution is used, backslash retains its literal
meaning except when followed by $, `, or \. The first backquote not preceded by a back‐
slash terminates the command substitution. When using the $(command) form, all characters
between the parentheses make up the command; none are treated specially.
Command substitutions may be nested. To nest when using the backquoted form, escape the
inner backquotes with backslashes.
If the substitution appears within double quotes, word splitting and pathname expansion
are not performed on the results.