Put quotes around the path and use POSIX file
to get a file object for the path:
tell application "TextEdit"
open POSIX file "/Users/UserName/Desktop/save.rtf"
end tell
You can modify the text of a document by changing the text property:
tell application "TextEdit"
set text of document 1 to text of document 1 & "aa"
end tell
It removes all styles in rich text documents. It also inserts the text as 12-point Helvetica in plain text documents, regardless of the default font.
Creating a new rtf file:
tell application "TextEdit"
make new document at beginning with properties {text:"aa"}
close document 1 saving in POSIX file "/tmp/a.rtf"
end tell
printf %s\\n aa | textutil -inputencoding UTF-8 -convert rtf -stdin -output a.rtf