A RIS file cannot hold PDF files. Instead, it has a specific tag that identifies where to find those PDFs so they can be pulled in to your bibliographic software (e.g. Endnote).
In Datarama, when you select RIS as the Report Format, you will notice that additional options will appear: the Export Filename to L1 field and Prefix path to L1.
In the RIS file, the L1 tag indicates where to find the link to the PDF files and the prefix path is the location to where the PDFs are located on your computer. A prerequisite to downloading the RIS file is first downloading the PDF files that are in your Project to a location on your computer.
The following link will provide more information about downloading your attachments from your Project: Downloading PDFs