It seems that both .pbcdoc and .pcb files are used for two different Printed Circuit Board formats and so it is more than likely that one can be converted to the other or vice versa. You will just need a dedicated PCB editor for this, for example Altium Designer.
Convert Printed circuit board description to Printed circuit board design file.
We found one software available in the database for this conversion.
Microsoft Windows software - convert pcbdoc to pcb on Windows
A PCB and FPGA design software
Convert from pcbdoc file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.