The filetype I handle most frequently is pdf. Here is a list of my most frequently used pdf-related actions.
pdftoppm -png input.pdf output
pdftk is another brilliant tool to operate on pdf file. The main thing to remember is cat output.
pdftk file1.pdf file2.pdf cat output outputfile.pdf
pdftk file.pdf cat 2-6 output outputfile.pdf
pdftk file.pdf cat 1-12 13-end output outputfile.pdf
You can also use pdftk to encrypt pdf files. See man pdftk. It’s amazing.