Originally Posted by eXpander_
This gives very poor quality results due to its internal conversion algorithms. To increase the quality of the output, it is better to read the input PDF files with a higher density (in dots per inch (DPI)) and then resize the output density back to common DPI, 96 for example. I usually use a input density of 600 (after many trial and errors), and then resize with 93.75% (if you dont resize you will get a huge output file).
convert -density 600 fileinput_1.pdf fileinput2_.pdf -resize 93.75% output.pdf
Have you actually tried and confirmed that you need the -density?
I am well aware of this switch when converting a .pdf to .jpg. Standard density is 50 dpi or so, not even 96 IIRC. However, when I tested this command I posted yesterday I left out the -density on purpose, and the merged file happened to be the same resolution as the originals.
This is my version in case it is relevant:
Version: ImageMagick 6.6.0-4 2010-11-16 Q16 http://www.imagemagick.org
Copyright: Copyright (C) 1999-2010 ImageMagick Studio LLC
Besides, a density of 600 even for a single PDF usually brings your computer down on its knees.