Cannot combine any of my pdfs

I just purchased this service so I could combine pdfs into one file (scanned documents, etc.). When I upload the files to combine them, "conversion" fails each time saying the format is not supported. Is this normal? I feel like I was duped into buying a useless service. Because now it says I can combine .docs, .jpgs etc, but not pdfs. When I look at the properties of my files in DC reader, it says they cannot be combined or pages cannot be extracted under the "Security" tab. My subscription is pointless if this is the case. Is there a workaround? How do I combine several pdfs to form one, multi-page pdf file?