Distiller 5 on Mac OS 9. Acrobat 6 Pro on Mac OS X .3.4

I often receive PDF files (ver 1.3) from clients and used to view the files from Acrobat 5. Everything is perfect.

After upgrading to Acrobat 6 Pro quite some time back, I notice that some of the PDF files (ver 1.3) has the text characters "I" and "L" shown as 'broken/dotted' lines on screen display. When printed, the "I" and "L" showed the same results as on screen display.

Not all the "I"s and "L"s is affected though, only certain parts of the file. To correct the defect, I had to get my clients modify the dtp doent ; to send the text to front and re-create a new PDF.

Is this a bug? And would there be a patch/upgrade to resolve this?
Anyone has a better solution?