I used adobe acrobat, tinypdf, pdfofficer, and pdf expert, and foxit pdf. Finally I decide to leave Foxit PDF on my iPad.
1). It is free comparing with PDFoffice, PDF expert, Adobe Acrobat
2). The function is similar to the PDF editor which charges money, and also better than TinyPDF.
3). The only problem I have is it is not synchronized well with iCloud. The Apple preview does not support the edition on Foxit PDF.
In all, it is the PDF editor that I finally choose to keep on my Ipad.
peweew about Foxit PDF Editor, v4.5.0