SmallPDF is a free, online service that gives you the ability to compress, convert, split, merge, modify, (and more) PDF files.
It's very easy to use. Select the function you want to perform from the home page and then just drag and drop your files. It's that easy. It supports converting images to PDF and PDF to Office and Office to PDF, along with much more.
You can even send them a PDF and they will print it and mail it to an address for you.
Here is a list of all of the functions.
JPG to PDF
PDF to JPG
PDF to Word
PDF to Excel
PDF to PPT
Word to PDF
Excel to PDF
PPT to PDF
Here are some More Resources for working with PDF Files (lots here, plus more listed below)
PDF Converter - free online PDF conversion
Lots of PDF resources - print, markup, convert and more
PDFBinder - simple tool to merge PDF documents into one
BabyPDF - Edit PDF documents for free
Crocodoc - markup PDF files for free
Fill Any PDF form - fill out, sign and send forms