All PDP++ objects can also be saved to a file and loaded back in from that file. Note that when you save an object that has sub-objects in it (i.e. a Layer, which has Units in it), those sub-objects are also saved in the file, and loaded back in as well. Thus, to save all the elements of a project, one only has to save the project object, which will automatically save all of its sub-objects.
There are two ways in which objects can be loaded back in: in one case, you load the file over an existing object. This corresponds to the Load Over menu item that you will frequently see in PDP++. This will simply replace the values of the existing object with those saved in the file. This also applies to any sub-objects. Note if the existing object has fewer of some kind of sub-object than the saved file, new sub-objects will be created as needed. However, the same is not also true if the existing object has more of these sub-objects--they are not removed to make the existing object exactly the same as that which was saved in the file. If you want to make sure the object is exactly as it was saved, open a new one, do not open over an existing one.
To open a new object from a saved file, use the Open in menu item (instead of Load Over). This is equivalent to opening a new item into a parent group object. For example, the project has a group that contains its networks. If you open a saved network file in this group, as opposed to over an existing network in the group, a new network will automatically be created (along with all of its saved substructure).
Some types of objects have default "extensions" that are automatically added on to the file name to identify the type of file. For example, projects are saved with a `.proj' extension. Further, some objects automatically compress the file after saving, (and automatically uncompress it before loading) in order to save disk space. This is because the file format objects are saved in is actually text-based and human readable. Thus, it is much more efficient to save large files in a compressed form. These files automatically get the compression suffix associated with the type of compression used (.gz for gzip, which is the default).