Recently, my wife want to merge some Adobe PDF files and I recommend her to use PDFsam Basic which is free.

The features of free PDFsam Basic

Merge

Merge PDF files, select the pages, merge bookmarks and interactive forms.

Split

Split a PDF file at given page numbers, at given bookmarks level or in files of a given size.

Extract from PDF

Extract pages from PDF files.

Rotate

Rotate PDF files, every page or just the selected pages.

Mix

Merge PDF files together taking pages alternatively from one and the other. The perfect tool if you have a single-sided scanner.