Haven't seen that before.
Try this....
FILE_EXISTS('c:\PDFs\'+ OL.UID +'.pdf')='Yes'
Or
Can you try building the entire filepath string first in say a SystemsSettings attribute and then use that and see if it makes a difference.
SystemSettings.MyFilePath='c:\PDFs\'+ OL.UID +'.pdf'
FILE_EXISTS(SystemSettings.MyFilePath)='Yes'