Ghostscript gives you the power to combine files, convert files, and much more, all from the command line.

It is easy to combine several input files into one combined PDF using Ghostscript:

 gs -sDEVICE=pdfwrite \
    -dNOPAUSE -dBATCH -dSAFER \
    -sOutputFile=combined.pdf \
    first.pdf \
    second.pdf \
    third.pdf [...]

Your input files don't even need to be PDF files. You can also use PostScript or EPS files, or any mixture of the three:

 gs -sDEVICE=pdfwrite \
    -dNOPAUSE -dBATCH -dSAFER \
    -sOutputFile=combined.pdf \
    first.pdf \
    second.ps \
    third.eps [...]

The combined.pdf file will contain the input files in the order given on the commandline. If you don't want the combined file to be PDF, but PostScript instead, you may want to use this:

 gs -sDEVICE=pswrite \
    -dNOPAUSE -dBATCH -dSAFER \
    -sOutputFile=combined.ps \
    first.pdf \
    second.ps \
    third.eps [...]

Should you for whatever reason want PostScript level 1 output, add a language level parameter:

 gs -sDEVICE=pswrite \
    -dLanguageLevel=1 \
    -dNOPAUSE -dBATCH -dSAFER \
    -sOutputFile=combined.ps \
    first.pdf \
    second.ps \
    third.eps [...]

The default PostScript language output level is 2. Using "1.5" is also supported, which is language level 1 with color extensions.

You can convert color input files into black/white or non-color/gray PostScript like this:

 gs -sDEVICE=psgray \
    -dNOPAUSE -dBATCH -dSAFER \
    -sOutputFile=combined.ps \
    first.pdf \
    second.ps \
    third.eps [...]

 gs -sDEVICE=psmono \
    -dNOPAUSE -dBATCH -dSAFER \
    -sOutputFile=combined.ps \
    first.pdf \
    second.ps \
    third.eps [...]

Should you for some reason need a series of single-page EPS files made up of pages from various input files, try this:

 gs -sDEVICE=epswrite \
    -dNOPAUSE -dBATCH -dSAFER \
    -sOutputFile=p%08d.eps \
    5page-first.pdf \
    7page-second.ps \
    1page-third.eps [...]

The resulting files will be nicely named as p00000001.eps .... p00000013.eps ...

But be aware, converting PDFs back to PostScript (or EPS), like the last 6 commands did, may loose some or much of the original quality. For example, PostScript can't handle transparencies directly (it fakes them by converting them into bitmap patterns), and converting such a PostScript file back to PDF will not restore the original transparency feature. Also, some other aspects of the graphic quality from the input PDFs may be deteriorated.

So in general, it's better to stay with PDFs and avoid roundtrip conversions to PostScript and back to PDF...

Should you need TIFFs or JPEGs from all pages of your input files, try this:

 gs -sDEVICE=tiffg4 \
    -dNOPAUSE -dBATCH -dSAFER \
    -sOutputFile=p%08d.tif \
    -r600x600 \
    5page-first.pdf \
    7page-second.ps \
    1page-third.eps [...]

 gs -sDEVICE=jpeg \
    -dNOPAUSE -dBATCH -dSAFER \
    -r600x600 \
    -sOutputFile=p%08d.jpg \
    5page-first.pdf \
    7page-second.ps \
    1page-third.eps [...]

Graphic gurus, check this out. To create color separations (CMYK), use:

 gs -sDEVICE=tiffsep \
    -dNOPAUSE -dBATCH -dSAFER \
    -r600x600 \
    -sOutputFile=p%08d.tif \
    5page-first.pdf \
    7page-second.ps \
    1page-third.eps [...]

We included an extra parameter in the last few examples to make the output resolution 600dpi, because we don't like the default 72dpi when it comes to pure full page image files. Now, you may be surprised: for each single page of the input files you automatically get 5 different files:

 p000000XX.tif
 p000000XX.Cyan.tif
 p000000XX.Magenta.tif
 p000000XX.Yellow.tif
 p000000XX.Black.tif

The *.tif file will be the biggest, since it contains a single 32 bit composite CMYK file (tiff32nc format). The four *.Colorname.tif files are not really colored (as one might think from their names), but in reality they are tiffgray files meant for creating offset printing plates for the respective separation in 4-color CMYK printing. If Ghostscript autodetected so called "spot colors" in the input files, these will get their own separation files, with a naming convention of *.s1.tif, *.s2.tif,... etc. (up to 64 different process and spot colors are supported).