I had so much trouble getting any tool to calculate it (I don't know how to get Oracle to use 64bit numbers and if you don't it overflows during the calculation) that I converted the number to a hexidecimal string and then performed the two's complement calculation by swapping each digit by its hexidecimal compliment.
Anyway - maybe you can use this as a starting point.
Re: Copy a file from DOCUMENTUM to PPM Server Path
PPM has no idea about the storage location of the file (it can't really - Documentum could be configured to use different storage strategies) - so it will not be able to provide a reference that can be copied using a unix copy.
If you use the provided link you would need to use a tool like wget to do an HTTP GET to retrieve the file.
This needs some mechanism to authenticate as the tool won't have an active session.