Because I like to underline the texts I find important after reading them, I also can't help but drag the mouse to underline when reading PDF files. Unfortunately, no line can be drawn at all. By chance, while using Foxit Reader software to read PDF files, I found that Foxit Reader can actually add annotations to PDF files!
Foxit Reader can add many different styles of annotations to PDF files: highlights, underlines, wavy lines, boxes, arrows, callouts...
If you like the powerful features of this software, hurry up and download it to experience the compact yet fully-featured Foxit Reader!
The download address for Foxit Reader is: http://www.pdfyueduqi.net.cn/