The industry standard software for working with PDF files is Adobe Acrobat; however, there are numerous PDF tools freely available on the Internet.
Step 1: Save the vector-based PDF files as raster images (TIF or PNG). The format of the raster images is important (300 ppi, monochrome). We recommend TIF files with LZW compression.
Step 2: Convert the raster images back to compressed PDF files.
Step 3: Merge the individual PDF files into a single multi-sheet PDF file.