How to display only pending pages on the tree

There is no such a setting for this. The content tree pages can be shown/hidden based on ACL permissions only. Your need is very custom - this means you would need to fully customize the entire Pages app UI - the content tree control and so on. It might be required access to the source code to do so.
I would maybe recommend developing a custom module where you will implement the logic you want using custom code and Kentico API. Or, adjusting the Pending pages app and its underlaying controls.

thanks for your reply, can you guide how to customize the pending pages app, is there a global event for it I can use or would be on CMS code customization

If you take a look at the Modules app and find the Pending pages in the User interface section (screen shot), you will see it is using this physical file: ~/CMSModules/MyDesk/WaitingForApproval/WaitingForApproval.aspx
and this page is using this control: ~/CMSModules/AdminControls/Controls/Documents/Documents.ascx which is basically an UniGrid. Modifying the system UI files is not upgrade/hotfix safe and could be overwritten back to defaults. So, either document these changes properly in the project documentation or, recommended approach would be getting inspiration in this module and create custom module and direct the users to this module.

Dear Jural

I'll implement a custom module, like the outdated pages

here is the scenario for me

I have a page type called "article", with a DateTime field called "NotifyMeOnDate", user set future date on this field to be notified later that this article needs to be updated

i see the documnets.ascx user control but it's too long as it's used for many modules can you provide a sample on how to make simple Unigird control with a document name column and edit action to take the user to the content tree, and with the filter of NotifyMeOnDate <= Datetime.now

Well, this is very hard advise. There are many other details and variables specific for your use case and needs I might not be aware of and which may change the result, the solution. So, I can tell you to do something but you will hit a road block because of some detail I did not knew. I would recommend using our consulting services to get hands-on help with the implementation and also suggestions tailored for your particular need and goals. Or, in general I can suggest to take a look at the UniGrid documentation and API examples on how to work with pages.