Opening already existing file:

File MenuàOpen (or) Ctrl+O

Many files can be opened at a time but one can be activated. 

Change between opened publications:

Ctrl+Tab (or) windows menuàselect publication

Close current publication: FileàClose (or) Ctrl+W

Save and Save As:

When you are saving a file for the first time, Save and Save As commands both work similar.

3 identifications of a file at the time of saving:

1. Location
2. Filename
3. Format
   1. Type of content
   2. Memory occupancy
   3. Editable or not
   4. App Support

When you are working with an already saved or re-opened file:

1. Save command saves the modifications in the original file
2. Save As command tries to save the modifications with another name, another location and another Format.

Save Newly created file: Fileà Save (or) Save As

Save modifications to saved file: FileàSave

Save another copy of already existing file: FileàSave As

Change dimensions of already opened publication:

FileàDocument Setup (or) Ctrl+shift+P

Change different preference settings of Page Maker:

FileàPreferencesàGeneral (or) Ctrl+K