From ArchWiki

A short howto on installing packages while offline. It is based on byte's post from this thread and uses KDE as an example.

Download the following when you got an internet connection available, and transfer the files to home PC

• ftp://ftp.archlinux.org/core/os/i686/core.db.tar.gz
• ftp://ftp.archlinux.org/extra/os/i686/extra.db.tar.gz
• ftp://ftp.archlinux.org/community/os/i686/community.db.tar.gz

Following steps will make sure you're working with up-to-date package lists, as if you ran pacman -Sy.

On home PC , do the following as root:

cd /var/lib/pacman/
mkdir -p sync/core
cd $_
tar -xzf {path-to-download}/core.db.tar.gz

cd ../..
mkdir -p sync/extra
cd $_
tar -xzf {path-to-download}/extra.db.tar.gz

cd ../..
mkdir -p sync/community
cd $_
tar -xzf {path-to-download}/community.db.tar.gz

pacman -Sp --noconfirm {package-name} > pkglist

Now open that textfile with an editor and delete all lines that are not URLs. Next, bring that list with you to a place where you have internet and either download the listed packages manually or do

wget -nv -i ../pkglist

in an empty directory. Take all the *.pkg.tar.gz files back home, put them in /var/cache/pacman/pkg and finally run

pacman -S {package-name}