I already read the thread how to merge epub files but Is there any way by which I can merge different pdf files into one. Earlier I have tried an app for this purpose but I lost all my pdf files after merging.
For serious work with PDF, you can't stay away from Adobe Acrobat… still…
You might also look at Nitro.
After a long search, I have downloaded PDFmate to merge pdf files. It works nice. It is free as well as from a verified publisher.
pdftk file1.pdf file2.pdf cat output combined.pdf
If you use OS X then the included Preview.app allows merging.
Open up one pdf file and then choose via the menus Edit->Insert->Page From File...
This prompts for a file - choose the second pdf and it adds all the pages from it to the original.
Kvisoft PDF Merger , available only for 30 day trial. Pdf combine also works.