Part of my work in book production involved pre- and post-publication changes to text and covers, mostly for reprints. It might be making a few small corrections in an easily-editable PDF, or going back into the InDesign files. Where neither of those methods are possible, I've used Illustrator or even Photoshop to carefully add, delete, and move text and other elements.

Example 1 - moving several figures and tables to different pages, moving text along with it:

Example 2 - editing the text colour from rich black to 100% black only. Because of how the file was set up (text was outlined), I couldn't make a global change in Acrobat and had to edit the colour on each individual page using Illustrator. This resulted in a massive file, but the client was able to use it.

Other notable editing projects in the past have included carefully adding/changing text in an old, scanned textbook; making changes to over 100 pages in a history of Ukraine; and surgically cutting/pasting letters in scanned pages to make minor corrections.