I'd try going into the Adobe program first and seeing if you can de-select .pln from the list of adobe recognised file extensions. This would usually be in options or preferences or something.
If this doesn't work, in Windows right click on any .pln file and select 'Opens with' > 'Choose Program' then select 'ArchiCAD' from the list. Select the 'Aways use selected program to open' checkbox to make this permanent.
It is often recommended that you always open ArchiCAD first, then select the file to be opened using the dialog box or the recently used list. I'd agree with that, and it's the method I use, but it's entirely up to you!
Hope you get it sorted!