Using and exchanging files in proprietary formats

Proprietary formats are widely diffused nowadays because they are used by a large number of publishing, image or text processing applications. A proprietary format encodes data in such a way that a file will only readable with the original software used to create it. On the contrary, an open format guarantees that a file can be read by any software written for that purpose.
The difference between open and proprietary formats might go unseen in cases of local use of files. By local use, we mean any personal use of files but also sharing of these files with other users who came to an agreement on which sotware should be used to read them.
As soon as the use is no more local and files are exchanged (by uploading them on the net, by publishing them, by sending them as e-mail attachments, i.e. any form of communication in which the sender and the receiver have no previous agreement on which software should be used for reading these files), the open vs. proprietary distinction becomes crucial.