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Brandon Rozek

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PhD Student @ RPI studying Automated Reasoning in AI and Linux Enthusiast.

Concatenating PDF files in Linux

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3 minute reading time

Every so often I need to combine several images into a single PDF. First, to convert an image to a PDF we can use imagemagick.

convert -quality 100 Image.png Scanned.pdf

To combine or concatenate multiple PDF files, we can use ghostscript.

gs -sDEVICE=pdfwrite \
   -sOUTPUTFILE=output.pdf \
   -dNOPAUSE \
   -dBATCH \
   input0.pdf input1.pdf input2.pdf
Flag Description
-sDEVICE Device used for processing the output file type. Use pdfwrite to write to a PDF file.
-sOUTPUTFILE Path to save the resulting file output.
-dNOPAUSE Disables the prompting and pausing at the end of each page.
-dBATCH Finishes interpreting after processing the inputted files

Alternatively you can use pdftk

pdftk input0.pdf input1.pdf input2.pdf \
	cat output output.pdf

Lastly, you can also use imagemagick. Do note, however, that this program often leads to larger file sizes.

convert input0.pdf input1.pdf input2.pdf output.pdf

Aside: Pixel Densities

One issue I came across is that the pages were of different sizes. This is often because the pages can be of different pixel densities.

To check run pdfimages and look at the 3rd to last and 2nd to last columns:

pdfimages -list filename.pdf 
page   num  type   width height color comp bpc  enc interp  object ID x-ppi y-ppi size ratio
--------------------------------------------------------------------------------------------
   1     0 image     613    77  rgb     3   8  jpeg   no         8  0  1071  1076 9456B 6.7%
   1     1 image    2692  3496  rgb     3   8  jpeg   no         9  0   329   329  418K 1.5%
   2     2 image     613    77  rgb     3   8  jpeg   no         8  0   915   919 9456B 6.7%
   2     3 image    2300  3016  rgb     3   8  jpeg   no        15  0   282   282  322K 1.6%
   3     4 image     613    77  rgb     3   8  jpeg   no         8  0   937   942 9456B 6.7%
   3     5 image    2356  3024  rgb     3   8  jpeg   no        21  0   288   288  150K 0.7%
   4     6 image    1686  2200  rgb     3   8  jpeg   no        27  0   204   204  622K 5.7%
   5     7 image    5100  7016  rgb     3   8  jpeg   no        33  0   600   600 1193K 1.1%
   6     8 image     613    77  rgb     3   8  jpeg   no         8  0  1104  1110 9456B 6.7%
   6     9 image    2776  3720  rgb     3   8  jpeg   no        38  0   339   339  231K 0.8%
   7    10 image     613    77  rgb     3   8  jpeg   no         8  0   939   943 9456B 6.7%
   7    11 image    2360  3072  rgb     3   8  jpeg   no        44  0   289   289  151K 0.7%

We can then use imagemagick to enforce a certain pixel density. The tradeoff being that the file size might increase.

convert -density 300 input.pdf output.pdf

If you happen to know a different way to enforce a pixel density that doesn’t have a file size increase tradeoff. Please get in touch.

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