right.....I tested it....I added one action: Save....
And last question(thanks for your patience! 
)
The following codes demonstrayed how I can combine two pdfs in one.
But again...I don't know how many files are there in my folder, I can't specify them. What should I do in this case? How to send parameters to the new array?
var doc = app.openDoc({
cPath: "/C/temp/doc1.pdf"
})
aFiles = new Array("doc2.pdf","doc3.pdf");
for ( var i=0; i < aFiles.length; i++) {
doc.insertPages ({
nPage: doc.numPages-1,
cPath: aFiles[i],
nStart: 0
});
}