How To Modify The Pdf File?

It depends if the PDF is image-type (e.g. Scanned document) or text-type (e.g. Produced from word processor software). It also depends if it is protected or not. If it's protected, don't bother. If it's not protected and text-type, just select the whole text (Control-A), copy / paste into e.g. Word, make the alterations, then create a new PDF. If it's image-type, you'll need to do OCR (later versions of Acrobat can do this, or find a standalone package, all the good ones cost money) to make it a text-type. Finally, Acrobat itself (full version, not just the reader) includes some tools to do minor edits.

If you just need add few words,you can import into photoshop,then you can edit.but if you need do huge work,i suggest you to convert it into word, i recommend simpo pdf to word,many people use it,you can have a try!