Best way to modify Excel files in C# for Windows Phone

Best way to modify Excel files in C# for Windows Phone

I`m looking for solution to modify excel files inside Windows Phone and
after that create a PDF from it:
Thats subject of my student project - Ive got personal expenses claim
(where employyes put their expenses) - thats excel file. I need to create
a WP8 app that can modify that excel file and after that create a pdf from
it.
Any suggestions ? I`m newbie in WP & Office integration - i dont know
maybe better idea is change file format from xls to some template files or
csv etc. ?