Sometimes when I open a project created by someone else, I get an error indicating the printer cannot be found and that a Default Printer will be used instead. What is causing this?
The map was created using a specific printer to which you do not have access. If you know you're sharing a map document with colleagues, you can make the map page size independent of the system printer. Under File > Page Set up, uncheck the box for ‘Same as Printer’. This will change the map document to be independent of the printer set up. The map originator must make this change; if you make this change as the recipient, it is already too late.
If you give a map document that's dependent on your printer setup to someone who doesn't have that printer, they'll get a warning message when they open the map. The map will then rescale to fit their default printer size and page orientation—usually 8.5x11 inches and portrait orientation. The result may differ considerably from the map you designed. If you make the page size independent of the printer and set an appropriate page size, you can prevent this problem.