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content/blog/concatenating-pdf-linux.md

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+---
+title: "Concatenating PDF files in Linux"
+date: 2022-12-18T18:38:44-05:00
+draft: false
+tags: []
+math: false
+---
+
+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`.
+
+```bash
+convert -quality 100 Image.png Scanned.pdf
+```
+
+To combine or concatenate multiple PDF files, we can use `ghostscript`.
+
+```bash
+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`
+
+```bash
+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.
+
+```bash
+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:
+
+```bash
+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.
+
+```bash
+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.