After editing the PageSizes.txt file to add a custom page size or change one of the built-in page sizes, every time I click on the "Arrange > Impose" action on the Switchboard, I get an error dialog box and the Impose action isn't shown. I have tried to uninstall and re-install pdfToolbox but that doesn't help.
The PageSize.txt file keeps all page sizes used by the Impose action in a very specific machine-readable format. Even though the file seems to contain only plain text, it defines each page size with a precise number of tab characters in between the different fields of a page size definition. If you edit the file, you have to keep to this format exactly or pdfToolbox will no longer be able to parse the file and cause the error dialog box to appear.
Uninstalling and reinstalling the software doesn't solve this problem because this file is installed in the user preferences. As it is assumed to be customizable by the user (you) the uninstaller doesn't remove the file and as a result the reinstall process doesn't reinstall a correct copy of the file.
First of all, find the PageSizes.txt file on your system:
Replace USERNAME with your actual user name on your machine and pdfToolbox X by the version of pdfToolbox you have installed. Alternatively you can also search by name for the PageSizes.txt file. In that case select the one in your preferences folder.
To fix the problem, you can do either of two things