combining pdf files

I have scanned multiple pdf files and want to combine them into one pdf file. I was sucessful once using preview by moving the thumnails all to on of the pdf files. The results was one pdf file with multiple pages that came from individual pdf files I had scanned. I tried that process again and the thumnails showed up as individual documents and not as combined pages in one file. I'm not sure how it worked before and don't know why it did not woked the second time. I have Adobe reader only which does not seep to allow me to add pages to an existing document.