This issue results from non ascii characters being used for a summary in the PDF. To resolve this error temporarily, follow this steps:
1) Browse to exports directory
PDF exports are saved per user. To find the corrupt message, browse to the exports directory for that user. The exports directory is located where Retain stores its archives.
2) Edit xml file
The error is in the XML file for a given export. Since the character was not properly encoded into XML, Retain fails to parse the XML document. The bad data is typically in the Title or Summary section of the XML file. Remove any non-ascii characters and save the file
Repeat for each xml file in the directory that has bad data.
Once the file is saved, the Exported Items Page can be opened and the error will no longer be present. If the issue remains then please contact support.