You can't use the document viewer to edit files.
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# Can I edit documents in the document viewer?

The document viewer can't be used to make changes to documents. You need to use the appropriate editing application for the type of file you want to change.

PDF and PostScript (.ps) files are not usually intended to be edited, but PDF editing software is available. Try LibreOffice Draw or Okular, for example. Furthermore, pdfarranger allows rearranging or removing pages and merging files.