how to undo pending changes of files that are unchanged?

This is different based upon your choice of version control system in TFS, so the above problem only exists for TFS' traditional version control system TFVC and not git which has been supported for a few years now.

For TFVS a partial workaround is to mark all files, right-click and Undo... and click Undo Changes button in the following dialog. This will prompt you for the files that are different (letting you choose No or No to All) but not for the ones that have no changes. Important: I write 'partial' because this approach does not work for file renames as the file is considered unchanged so the rename will be undone.

Yes,there are no Power Tools for Team Foundation Server 2017. You could use below workaround:

Right-click on your project ,select undo checkout, then just click okay, or whatever confirmation is left...

Then, while undoing checkout, for every file that has REAL changes in it, a prompt will ask you to confirm the check out for that file... simply click "No to All". Visual Studio will know if the checked out file has changes or none.

WARNING: This method also removes new files, i.e. files that are not yet checked in to TFS. If you want to keep these files then simply exclude them from the set of files you "undo".

You could also take a look at this question: his question Files listed as pending changes when doing merges that didn't change?