The best way to modify and edit a PDF file is to use a PDF editor to complete the operation.

The operation method is as follows:

1. After opening the PDF editor, open the software and select Open-Browse to open the PDF document to be edited.

2. After opening the document, use Start-Edit Content-Text Only in the toolbar.

3. Then select Start-Underline in the toolbar.

Step 5: Next, select the text content that needs to be underlined, which can be underlined directly.