Thanks for your reply! Actually, there does appear to be some connection; ConvertIFD.exe, which was installed under LCD 7.1, was desiring that PDF.ICF file, which is under the OD 5.6 directory structure. However, your advice to uninstall 5.6 and put on 5.5 was sound, and now the situation has been resolved and I can properly open the IFDs. Thanks again.
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For people wondering about opening IFDs in 7.1 and having an "XML parsing" error, I had that too in many of my IFDs. For me, at least, the solution was making sure that the description of the IFD (where some comments have been stored) did not have an empty space at the end and no illegal characters. Cleaning that up allowed me to open the docs.