If you received a .p7s file and cannot open it, you are not alone. These files are commonly used for signed email attachments and official documents.
A .p7s file is not a normal file. It is a signed container that may include PDF, Word or other documents inside.
Most Windows systems do not support .p7s files directly. That is why double-clicking the file usually does not show anything useful.
The easiest way is to use a viewer tool that can extract the real content inside the file.
You can use this tool:
P7S Viewer - Open .p7s files