Intel Mac :: Preview Error Message <doc> "can Not Be Saved Because It Does Not Exist"

I have had a long standing problem with Preview. I know it is odd but around closing time every day, two of my computers both running 10.6.8 get error message when trying to save a new document. The document will be open on my screen and when I go to save it I get the error message that the file can not be saved because it does not exist. Some background that may be relevent: 

The documents that I create all day long (for at least 6 years) are output from a 4D database. I generate a report from the db and it opens in Preview as a PDF file. All day long I have no issues with creating and saving these docs. Then around 4-4:30 when many people are getting ready to leave two of our computers (one iMac 3.06GHz Intel Core i3 and an older macMini) have this problem. All you have to do to fix it is quit Preview and regen the doc. Unfortunately we often lose a lot of work because of it. I've tried anticipating when it will happen and restart Preview before creating the new doc but that only works occasionally.  

I suspect that it has something to do with people logging out (I am usually the last to leave) and perhaps some shared application is effected. Everyone does have their own copy of Preview and we all use separate 4D clients. I have searched the internet many times. I have found some similar problems that people have had with this error but not related to my own. Unfortunately both Preview and "the file can not be saved because it does not exist" are very general terms that bring up all kinds of unrelated things. 

I was going to try to delete the Preview Prefs file and start again but I can not find a plist for preview or any other file that is clearly the prefs. Searched the internet for that too but did not find it.

Info:
iMac, Mac OS X (10.6.8)