The PacletInfo.m is a descriptor file for Wolfram Language applications. The Wolfram Workbench provides a special multi-page editor that helps you to work with it. When you open the PacletInfo.m you see an overview page, with tabs to navigate to the other pages. An example is shown below.
If you change anything in these fields, you will need to save the editor. You can learn about the fields in the PacletInfo.m contents section.
You can visit the other pages of the editor by clicking on the respective tabs.
The Extensions tab lists all the extensions in the PacletInfo.m. Typically, this is just going to list the Documentation extension. A sample follows.
The Documentation tab shows the documentation you have written for your application. An example is shown below.
From this page you add or remove documentation pages. There are also special tools for creating symbol pages and an overview page.
If you find that this page does not show a documentation outline you should confirm that you have set the Documentation Location in the project properties. If this is not set properly the PacletInfo.m editor will not be able to find your documentation.
The details of this page are described in the section on the documentation extension.
The PacletInfo.m tab shows the raw source of the file. You can edit the contents of the file directly, but typically this is not necessary since the other tabs provide a way to set it up.