RIS is one of the accepted file formats when uploading references into your DistillerSR Project. DistillerSR will check for tags in the file. The RIS file must begin with the tag TY and end with the tag ER - if this format is not followed, the reference (or file) will be skipped and not uploaded.
DistillerSR is also looking for an ID tag, as it will use this to assign RefIDs to the references. If this tag is not present in your file, you must provide a "Sequential RefID starting at" in the Assign RefID section during the upload.
To check tags in your RIS file, open the file in Note Pad. Anything before TY and after ER should be deleted and the file can be uploaded successfully.