Combine two separate 8.5 x 11 PDF documents into one 11 x 17 PDF document?

Question

I have some 8.5 x 11 PDF documents that are the "left" and "right" pages of what should be a 11 x 17 document. I am looking for a way to combine/stitch these files together. Either a Windows or Mac solution is acceptable.

Most solutions I find seem to merge these two single page documents into a two page document, and not combine them side by side into a 11 x 17 larger document.

Additionally, I'd prefer the solution to be "free" and "easy": "Free" meaning I'd rather not buy a commercial product and "easy" meaning not having to download and compile a set of command line tools.

Solution

1. Merge all pdf files to single pdf file
2. you can achieve this scenario in Acrobat Pro/Reader 9
3. open your Pdf file in Acrobat Pro/Reader 9
4. Press ctrl+p to open printing option
5. Use booklet option to merge pages as left and right side like ([1,2][3,4])
6. set the following parameter. Please note that name should Adobe pdf and not your printer
7. Once you got your file as booklet you can resize it as per your desire

Hope this helps!