It is possible to change the title of the 3D PDF of the 3D Textured Mesh but not directly in Pix4D desktop. An alternative would be the following:
1. Download and install Foxit Reader.
2. Open the 3D PDF file of the 3D Textured Mesh with Foxit Reader. It does not matter whether the model can be visualized or not.
3. On the Menu bar, click Comment.
4. In the section Drawing, click the tool to draw a rectangle.
5. Draw a rectangle around the part to be removed.
6. On the Menu bar, click Comment Format of the Drawing Tool.
7. Adjust the style, e.g. no border and white color.
8. On the Menu bar, click Comment.
9. In the section Typewriter, click the button Typewriter.
10. Draw the box, move it and enter the desired name.
11. On the Menu bar, click Comment Format of the Free Text Tool.
12. Adjust the style of the text.
13. Click File > Save.
14. Close Foxit Reader.
15. Open the 3D PDF file of the 3D Textured Mesh with the usual PDF application supporting 3D data.