PDFpen supports documents with file attachments. To link to another document, see Links.
Read and Extract File Attachments
View an attached file by double-clicking on it in the sidebar.
Right-click on the file to extract it and save it or to preview the contents.
Adding and Deleting File Attachments
(Advanced feature of PDFpenPro). See Adding File Attachments.
Note: Not all PDF apps support attachments. A PDF re-saved in an app without support for this feature will erase the annotations. This includes Preview in El Capitan and earlier.