Issue Description
Archicad 10 through 26 will open Archicad PLN files going back to version 8.1. For files older than 8.1, you will need to open and save them in Archicad 10. Then this newly saved Archicad 10 file can be opened until version 26. Archicad 27 and above can open PLN files back to version 17. If you want to open older version files in the recent versions, first you have to save a copy from a version between Archicad 17 and 26.
!IMPORTANT! Due to changes in macOS, the file converter mentioned here no longer works on recent versions of macOS. Please contact Technical Support if you use macOS and let us know you have files you wish to have converted to a newer version that can be read by recent versions of Archicad.
Solution
Archicad 10 File Converter works on all recent versions of Windows. It worked on macOS 10.13 and below. Use this converter to up-save any 6.5, 7.0, or 8.0 PLN file to version 10.
1. Download the Archicad 10 USA for Windows package from here.
2. Once downloaded, double-click the ZIP file to extract it into a folder.
3. You can keep this folder anywhere on your system.
4. Open the folder, then locate and double-click the Archicad.exe file to launch Archicad 10.
5. Choose Open a project and click Browse to locate the file you wish to open.
6. Open the file, and once past any warning dialogs.
7. Choose File > Save as... and add a suffix, such as "v10", to the file name so that you keep your original file intact. Example "Smith Residence v10.pln".
8. Now close Archicad 10, and you may open the re-saved copy of the file in versions of Archicad until 26.