Question: How can I merge several `.png` files into one PDF file in Unix?

Answer: 
If I want to merge some scans to one PDF file, I do this:

```
pdfjoin --a4paper --fitpaper false --rotateoversize false scan01.png scan02.png

```

This gives you a PDF document with DIN-A4 page size, where every `png` file is centered on it's own page. Images that are too big for one DIN-A4 page are resized proportionally to fit on one page. Smaller images are not resized (not made bigger).

You have to name all `png` files on the command line, but you can also use wildcards to eg merge all png files in the current directory:

```
pdfjoin --a4paper --fitpaper false --rotateoversize false *.png

```

The `pdfjoin` command is part of PDFjam as mentioned in the answer by Jeremiah Willcock. So you will most likely have to install a package named `pdfjam` or `texlive-extra-utils` with your distros package manager. PDFjam is able to use `png` files as input since Version 2.07, released in 2010-11-13.
**ImageMagick**’s [***convert***](http://www.imagemagick.org/script/convert.php) tool is my preference.

> 
> The convert program is a member of the ImageMagick suite of tools.
> Use it to convert between image formats as well as resize an image,
> blur, crop, despeckle, dither, draw on, flip, join, re-sample, and much more.
> 
> 
> 
> ```
> convert [input-option] input-file [output-option] output-file`
> 
> ```
> 
> 

If you want the image files (and thus, their quality and file size) unaltered, and just put a PDF container around them:

```
convert In.png In-2.png Someother-*.png Result.pdf

```

In case you want to have a smaller file size, and you are okay with a loss in quality, you can convert them to the JPEG format first. (ImageMagick also supports changing the PNG compression level, but usually your input files are already using the highest level.)

```
convert 1.png 2.png -compress jpeg -quality 50 Result.pdf

```

Use a value between 0 and 100 for the [`quality`](http://www.imagemagick.org/script/command-line-options.php#quality) option.

Alternatively, you can reach a lower file size (and quality) by resampling the images to a certain resolution.

```
convert *.png 2.png -resample 300 Result.pdf

```

The value for [`resample`](http://www.imagemagick.org/script/command-line-options.php#resample) refers to the number of pixels per inches. ImageMagick reads the original density from EXIF part of the input images, falling back to 72 dpi. You can use the [`density`](http://www.imagemagick.org/script/command-line-options.php#density) parameter to set a custom resolution for the input images.

You can of course also combine the `compress`, `quality`, and `resample` parameters.