/ always starts at the root of your current drive. What *that* means depends on your OS!
You'd get the path from your current file by something like
currPath = app.activeDocument.path;
.. then you can add your temp folder with
embedDoc = new File(currPath+'/temp/testfile.pdf");
.. but *only* if that sub-folder actually exists in the path you stored the file in. The actual code depends entirely on your workflow, I guess.