Incorrect "date creation/modified" moving pdf files from older MacOs

Recently,

locking to the property of a pdf files, I noted that the date of recording and

last modification was erroneously recorded in a pdf file.  Checking in other files, it seems that the

problem is connected to the older MacOs version (I am using a snow leopard): when

you open and close pdf file, the system modifies the date of last modification;

when the file change folder, the system update the date to the today date…

Do you have

any suggestion / experience on the issue