(I don't have a Windows setup to test with, but assuming it just fills in the raw path when you drag-and-drop files, what I suggest here should work.)
Everything entered on a shell command line -- including arguments to your script -- need to conform to shell syntax, which assigns special meaning to backslashes, spaces, and a bunch of other characters you might find in a Windows file path. In order to be passed through to the script as literal characters, they must be quoted or escaped as per the rules of shell syntax.
But there is another way... Rather than passing the file path as an argument, you could have the script read it as input after you start the script. Since the script would be reading it directly, the script can control what syntax is or isn't needed. In particular, it can use read -r, where the -r means "raw mode", which doesn't mess with backslashes. Something like this:
#!/bin/bash
id=user
pc_ip=xxx.xxx.xxx.xxx
pc_port=xxx
read -r -p "File: " filepath
scp -P "$pc_port" -r "$id@$pc_ip:$filepath" .
Note that I also switched to lowercase variable names (lower- and mixed-case variable names are safer, because a bunch of all-caps names have special meanings that can cause trouble). I also double-quoted all the variable references, to avoid weird parsing of spaces and maybe other things (you should almost always do this).
You could also make the script accept either a command-line argument (which would have to be quoted/escaped) or if that's not supplied, take it as input instead:
...
if [[ -n "$1" ]]; then
filepath=$1
else
read -r -p "File: " filepath
fi
...
