1. Open and run xunjiePDF editor, and open the PDF file that needs to be modified in the editor.

2. After opening the file, select the document in the menu bar of the editor, then select the background in the document. In the background tool, there are Add, Delete All and Manage. Click Add options.

3. After clicking Add, you can choose a color background or choose to add a background image on the added page, and then set it in the page range. After setting it, click OK.

4. You can also select the management option in the background tool. After clicking the management option, you can add or edit the background in the file on the management page. You can also choose to delete or edit the background that is not needed in the file. Delete all.

5. After modifying the file, remember to save the file. Click File in the menu bar to save or save as.