There are several free utilities for merging PDFs into one document. For a long time, Ghostscript was enough for me:

gs -q -dNOPAUSE -dBATCH -sDEVICE=pdfwrite \\
    -sOutputFile=collated.pdf file1.pdf file2.pdf

Today I hit several PDFs with complex table formatting. As far as I could tell, gs (and pdf2ps) were rasterizing these documents into embedded bitmaps during processing. This dramatically increased file size and processing time. Some searching brought up this page and a comment suggesting the pdftk utility.

pdftk file1.pdf file2.pdf cat output collated.pdf

pdftk ended up producing much smaller files (far closer to what I would expect based on starting file size) at very fast speeds. And watermarking, oh how I searched for a utility that could watermark.

Thanks, pdftk team.