When loading plans into Cubit, it is important that the plans do not have any security restrictions on them, as this may result in the plans not being able to be loaded.
Attempting to load plans with restrictions on them will result in Cubit displaying a message telling you the PDF is not valid, followed by an error message.
Fig.1 Cubit Error Messages
A document that has restrictions on it, will also display a permissions error when attempting to open the PDF in Adobe.
Fig.2 Adobe Permissions Error
If you encounter plans that are restricted, then you will need to ask your architect to remove the restrictions from the plans.
After doing so, the plans should load into Cubit normally.