Matrix will only accept pdf files as Attached Docs/Supplements. It will not accept Word docs or any other file type.
Also, make sure that there is only one period/dot in front of the file extension. If there is more than one, Matrix will allow you to add the file, but it will not see it as a pdf file when you try to open it.
In the example below, the file on the left was saved as 128 Butlertown Rd Disc.pdf- it downloaded and opened correctly. The file on the right was saved as 128 Butlertown Rd Disc..pdf (with a second dot/period)- it is not recognized as a pdf and therefore will not open: