You will have to download the translator form mentor graphics if you do not already possess it. If you are truly going to convert this database from pads to expediton you will have to also convert library data: Parts, Cells, symbols, (symbols only if you convert schematic to dx or capture) and padstacks.
To do this conversion correctly is not a menial task. You will certainly want to educate yourself on process by reading documentation.
Finally, The Pads PCB file is a binary document that contains all information. The expedition PCB file is only an ascii file containing pointers to project ascii file and other data. These are 2 completely different data structures.
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