I THINK pdfscissors might help or work. You give it the pdf file and when it opens you can select the columns. It will then cut those columns like cropping a picture and reformat them into a new pdf. You can select both columns, first the left and then the right at the same time so that it will interweave them into one consecutive pdf file. I do not think it works with protected pdf files.
I took a breadmaker manual with 3 columns and converted it into ONE column output.