Removing encryption from (legitimate!) PDF files on Ubuntu

Many service providers have started encrypting the statements that they send you. While at some level, it does add some amount of security when the path to your inbox is not very secure. However, it is sometimes a major pain when you want to archive your emails. This is because every provider has decided on a different secret to encrypt your PDF. So if one day you wish to access a statement of your phone bill from three months back, you have to look up the bill from your archive and read the mail to find out what they used to encrypt it.