Hi Matt,
Thank you for your interest in Syncfusion products.
I am afraid that I am unable to reproduce the issue "while replacing the string with document it throwing an exception", its working fine in our end.
Please refers to the below sample:
http://www.syncfusion.com/uploads/redirect.aspx?file=FindandReplace_c68f9916.zip&team=supportPlease have a look into the sample and try to reproduce the issue in that sample or send your sample to us to reproduce the issue. This would be very helpful for us to investigate more on this issue and provide a solution.
Note: In subject you have mentioned .doc and in code you have used .docx. But issue is not reproduced in both the formats.
Best Regards,
Bhuvana