Using a scanner in arch is pretty perplexing at first, especially if you want to try it from the command line. Here are my steps to end up with something that works.
First install the sane package. Sane provides a command line tool for using scanners.
sudo pacman -S sane
If the scanner is a hp brand, setting up hp-lip also helps out.
sudo pacman -S hplip
This gives features that help setting up the printer or scanner easily like hp-setup. Using it and the various prompts, one can set up the printer and scanner easily.
Assuming the device has already been set up, find a list of devices using:
My output looks like this:
device `v4l:/dev/video0' is a Noname USB2.0 HD UVC WebCam: USB2.0 HD virtual device device `hpaio:/net/HP_LaserJet_Pro_MFP_M127fw?ip=192.168.1.27' is a Hewlett-Packard HP_LaserJet_Pro_MFP_M127fw all-in-one
To determine options for my device I have to do:
scanimage --device "device name" -A
Which in my case is:
scanimage --device 'hpaio:/net/HP_LaserJet_Pro_MFP_M127fw?ip=192.168.1.27' -A
This in my case outputs:
All options specific to device `hpaio:/net/HP_LaserJet_Pro_MFP_M127fw?ip=192.168.1.27': Scan mode: --mode Lineart|Gray|Color [Lineart] Selects the scan mode (e.g., lineart, monochrome, or color). --resolution 75|100|150|200|300|600|1200dpi  Sets the resolution of the scanned image. --source Flatbed|ADF [Flatbed] Selects the scan source (such as a document-feeder). Advanced: --brightness -1000..1000  Controls the brightness of the acquired image. --contrast -1000..1000  Controls the contrast of the acquired image. --compression None|JPEG [JPEG] Selects the scanner compression method for faster scans, possibly at the expense of image quality. --jpeg-quality 0..100 [inactive] Sets the scanner JPEG compression factor. Larger numbers mean better compression, and smaller numbers mean better image quality. Geometry: -l 0..215.9mm  Top-left x position of scan area. -t 0..296.926mm  Top-left y position of scan area. -x 0..215.9mm [215.9] Width of scan-area. -y 0..296.926mm [296.926] Height of scan-area.
And using this information I can scan the document. For example to output to a tiff file, I'll do:
scanimage --device 'hpaio:/net/HP_LaserJet_Pro_MFP_M127fw?ip=192.168.1.27' --format=tiff --mode=color --resolution=75 > 24.tiff
This sets the mode to color and the resolution to 75. These options are found from the previous output.
To get the output into a pdf file, I use tiff2pdf. If I have multiple files that I scanned, I add them into a single file using tiffcp.
tiffcp 1.tiff 2.tiff 3.tiff output.tiff tiffpdf -j output.tiff -o output.pdf
A more advanced example is:
tiff2pdf -p letter -j -q 75 -t "title" -f -o title.pdf doc.tiff
However using tiff2pdf occasionally fails because libtiff has set and upper limit of memory to be used by libjpeg. So if I have really large tiff files, I'll most likely get the following error:
JPEGLib: Backing store not supported. tiff2pdf: Error writing encoded strip to output PDF sm.pdf. tiff2pdf: An error occurred creating output PDF file.
I could not find a fix for this. So I just made sure my tiff files were less than 10MB. If a file became more than that size, I reduced the resolution of scanning. Also while combining files using tiffcp, if the eventual tiff file is too big, I convert the individual tiff files into pdfs and combine the pdfs using the following command:
pdfunite 1.pdf 2.pdf output.pdf
And that is how I scan document in my arch setup.