Using Microsoft Excel 2013, If I add a "comment" to a field and then open that file in Excel Viewer, I get the "Open File Dialog" window and can't open the commented excel file. 

Why can't commented Excel files be opened in Excel Viewer?

Thanks,

Scott

I test you issue in my own environment, and I get the same result as yours. I find excel viewer cant open the .xlsx file with comment. Then I save the file as .xls formatting, I find excel viewer can open .xls file with comment in excel 2013. As shown in the following figure.

I notice that, Excel Viewer was published in 2008, and it applies the version of office is Microsoft Excel 2010\2007\2003\2002\2000\97

I didnt find it applies excel 2013, so I test this issue in excel 2010 with .xlsx file. I find it works fine in excel 2010. As shown in the following figure.

With above tests, I suppose this issue might be occurred by compatibility between excel 2013 and excel viewer.

I suggest you can save the file as .xls as a workaround to help you solve this issue.

Hope its helpful.

Regards,