This article is especially for the one who want to free merge their multiple PDF files into one with best quality. Just recommend four freeware and one shareware for windows user.
PDFill PDF Tools
Clean and easy to user interface, include most features such as merge, split, reorder, rotate, header and footer with page number, watermark, encrypt and decrypt, convert PDF to images or vice versa
However, installer bundles with PDFill PDF Editor which is not free.
PDFTK Builder
Merge and reorder of pages at one go, supports split, rotate, background stamp, encrypt, etc. However it can not supports windows 7.
Gios PDF Splitter and Merger
Gios PDF Splitter and Merger is a handy tool that will help you merging and splitting PDF documents with ease. Version 2.0 supports also merging to PDF of Jpeg images(scanned documents).
Merge PDF
Utility to concatenate PDF files into one, its use is very simple. But it is not a English software.
True it is in french, also easy to use.
Click open file simbol, select the files you want to merge(you must add them at a time)
Click on discette to give it a name and destination
Click the lightning simbol down left. 
Best PDF Merger
The PDF Merger is a great software for merging multiple PDF files together without Adobe Reader or Acrobat. Easy to use, you can control, manage or batch transfer your PDF files more efficiently on windows 7, vista, xp, 2000.
It also provides special merge methods that allow you to modify the range of merger to make the merger more effective, you can choose certain useful pages from PDF files for merging and customize a unifying name for the imported PDF files.
Convert PDF, Create PDF and Edit PDF as ease
