Rahul Ba wrote:We sent email with attachment. Attachment can be in pdf, xls, csv, txt formats. We want know how many user opened the file.so need some mechanism.
You can't.
Unless you provide them an application that is the only one they can use to open each of those types of document, and then that application informs you it has opened file X.
Of course then you'd get informed of every file they opened of those types, whether emailed or not.