Combine files stuck on checking for security settings?

Adobe Acrobat X Pro 10.1.1 on Mac OS X 10.6.8

I've been trying to combine 2 pdf files (scanned to pdf using Preview on Mac) in Adobe Acrobat. However, it keeps getting stuck on checking for security settings for a long time (see screenshot below).

I checked the security settings on each pdf file and they both are showing no security.

I opened the original file using Preview, open the Print window and saved it to PDF under different name and still I can't combine the files. Is there any other way to get this done? I've been searching on Google and this forum for hours and found no solution to my problem.

Thanks.