What version of MODX are you using? FileAttach doesn’t seem to be compatible yet with MODX 3.
What PHP version?
Does this problem occur for all the download files, or only for this specific PDF-file?
Are there any errors that get added to the MODX error log when you call the FileAttach snippet?
Change the system setting fileattach.files_path to something like mydocuments/. (Put a / at the end. No slash at the beginning.)
Then add the folder “mydocuments” to the file system.
When you upload new files, they get put into the “mydocuments” folder and the [[+url]] placeholder in the chunk should now have the correct value.