PDF files are invisible after they are replaced with the existing ones.

I just upgraded my Adobe illustrator. After upgrading, whenever i save as a pdf file and replace it with an existing one. Its not visible on the folder by it asks for "replace" when you save it in to the same folder. But its not visible in the folder. Hence you have to save files with new name each time. My machine's configuration is- OS X V Version 10.9.5

Processor - 2x2.8 Quad -Core Intel Xeon

Memory: 13GB

Appreciate your help