Password Protected

I recently recievd a copy of my bill from the local utility company as a PDF attachement to an email. when I tried to open the PDF it rendored the "password" box asking me to enter a password. I called the utility and I was told they do not passowrd protect copies of bills they send to customers; this makes sense. I went back and checked previous bills I received from them as a PDF and they all ask for a password. I have Acrobat 11 2017 installed locally, I do use the password funtionality in Adobe Acrobat to protect documents I store both on my machine and in the cloud but the password I use did not work on the PDF my utility company sent me. I have seen this phenomonem before but never fiogured out what was going on. I have the MS Outlook client installed on the same machine with no add ins or extensions from Adobe. Does anybody know what is causing some PDF files to ask for a password when the sender never added a password to the document?