gs is likely already installed.
pdftk can be installed using sudo aptitude install pdftk. pdftk is not recommended because of its old and outdated PDF engine.
pdfjam is part of the LaTeX distribution (sudo aptitude install texlive-full). However, pdfjam is not recommended either because hyperlinks are not preserved.

So my advice is to go with gs.

Other commands exist such as pdfseparate and pdfunite. They are very good but the output files are quite heavy, compared to those obtained using gs.

Convert a PDF to multiple PNGs (one image per page)

convert input.pdf -density 300 -background 'white' -alpha remove output.png

Add a password to a PDF file

pdftk input.pdf output output.pdf userpw <password here>

Reduce size/quality of a PDF file

ps2pdf -dPDFSETTINGS=/ebook input.pdf output.pdf

/ebook can also be replaced with /screen for further reduction.

Extracting pages

# Extracts all pages from 1 to 5
gs -sDEVICE=pdfwrite -dNOPAUSE -dBATCH -dSAFER -dFirstPage=1 -dLastPage=5 -sOutputFile=output.pdf input.pdf

# Extracts all pages from 1 to 2 and 4 to the end
pdftk input.pdf cat 1-2 4-end output output.pdf

# Extract only pages 1, 2, 4 and 5
pdftk input.pdf cat 1 2 4 5 output output.pdf

# Split each page into a file
pdftk input.pdf burst
# OR
file=input.pdf
pages=$(pdfinfo "$file" | grep "Pages" | awk '{print $2}') 
echo "Detect $pages in $file";
filename="${file%.*}";
for i in $(seq -w 1 "$pages"); do
    pdftk "$file" cat "$i" output "$filename-$i.pdf";
done;

Editing one page

Use Gimp (import with 300-dpi setting).

Merging several PDF files

gs -sDEVICE=pdfwrite -dNOPAUSE -dBATCH -dSAFER -sOutputFile=output.pdf input1.pdf input2.pdf input3.pdf
pdftk input1.pdf input2.pdf input3.pdf cat output output.pdf
pdfjam input1.pdf input2.pdf input3.pdf -o output.pdf

Convert one or several JPG files into a single pdf

First, in /etc/ImageMagick-6/policy.xml, comment out the last 6 lines:

<!-- <policy domain="coder" rights="none" pattern="PS" />
<policy domain="coder" rights="none" pattern="PS2" />
<policy domain="coder" rights="none" pattern="PS3" />
<policy domain="coder" rights="none" pattern="EPS" />
<policy domain="coder" rights="none" pattern="PDF" />
<policy domain="coder" rights="none" pattern="XPS" /> -->

Then:

convert image1.jpg image2.jpg output.pdf

Set the PDF size to A4 (210 x 297 mm)

Make sure your image has a resolution of 1050x1485 pixels. Then:

convert input.jpg -density 50 -units pixelspercentimeter output.pdf

Or, for a 2100x2970 pixel image:

convert input.jpg -density 100 -units pixelspercentimeter output.pdf

If the image is of a different resolution, use this:

i=100; convert input.png -compress jpeg -quality 70 \
  -density ${i}x${i} -units pixelspercentimeter \
  -resize $((i*21))x$((i*29.7)) \
  -repage $((i*21))x$((i*29.7)) output.pdf

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