The problem turns out to have been the colons - since my samba share doesn't allow filenames with colons, it would give this error. Removing the colons from my filename results in it working perfectly:
scanimage -d "pixma:04A91746_57E9C3" --resolution 300 --format tiff > $(date +%Y.%m.%d-%H.%M.%S).tiff
If anyone is curious about the completed script for scanning and organizing documents, here it is:
#!/bin/bash finish="-1" page=1 date=$(date +%Y.%m.%d-%H.%M.%S) while [ "$finish" = '-1' ]; do echo -e "\e[1m\e[32m==>\e[0m Scanning page $page..." scanimage -d "pixma:04A91746_57E9C3" --resolution 300 --format tiff > $date-$page.tiff while true; do echo -en " \e[1m\e[34m->\e[0m More pages? [y/n] " read answer case $answer in [Yy]* ) page=$((page+1)); break;; [Nn]* ) finish="1"; break;; * ) echo -e " \e[1m\e[34m->\e[0m Please answer tes or no.";; esac done done echo -e "\e[1m\e[32m==>\e[0m Converting $page page(s) to PDF..." convert $date-*.tiff $date.pdf echo -e "\e[1m\e[32m==>\e[0m Cleaning up..." rm $date-*.tiff
I just cd into whatever folder (receipts, medical, etc) and run scanfile. Result is a timestamped pdf of all pages scanned in the current directory.