Pages documents

The file browser within Numbers, Pages and Keynote Apps will intentionally only allow access to files created by - or compatible with - the corresponding App. Files that are not compatible with the App are shown as a grey “ghost” icon.

If instead you use the full Files App, you’ll be able to browse and access all files and file types.

When using the file browser of each of the core Apple Apps, you may have missed that they are colour coded. This colour keying is intended to provide a visual cue as to which file browser you are using...

• Pages App: White background with Light Grey sidebar, title text and functional icons in orange
• Numbers App: White background with Light Grey sidebar, title text and functional icons in green
• Keynote App: Black background with Dark Grey sidebar, title text and functional icons are blue
• Files App: White background with Light Grey sidebar, title text and functional icons in blue

Once this distinction is recognised, it should become immediately obvious which of the core Apps (and associated file browser) is being used.

I hope this helps to answer your question.