That file extension is usually associated with Adobe products, in most cases Acrobat. What is the file name? It is also not an executable file, except if it is used by malware. You should note that I have never seen a malware file using that extension in the many years I have been doing this kind of thing. If it scans properly with your antivirus and MBAM, it is highly unlikely to be malware. If you right click on the file and choose properties, you can usually find a tab in the panel that opens that may tell you what program developer it is associated with, although that is not always present. To scan the file, right click on it and your antivirus should have added a scan with line. MBAM always adds a similar line. Simply click on those lines, and the file will be scanned for you and that will alert you if it is somehow related to malware.