It is suggested that you use Python modules instead of pdftk to manipulating PDFs for several reasons. First, even though pdftk is a great command-line tool, it is hard to remember its syntax. On the contratry, Python code is easy to read and understand (even though it is more verbose). Second, it is not easy to have pdftk installed and configured on macOS. Please refer to PyPDF2 Examples for a simple example of extracting PDF pages using the Python package PyPDF2. If you need to edit PDF pages, please refer to Editing PDF Files for possible tools. If you do have to stick with pdftk, below are concrete examples on how to use it.

1. Fill in forms in an I-9 doc.

   # dump fields in the form (optional, for human examination only)
   pdftk i9.pdf dump_data_fields > fields.txt 
   # generate a FDF data file
   pdftk i9.pdf generate_fdf output data.fdf 
   # after fill in fileds in the FDF file, run the following command
   pdftk i9.pdf fill_form data.fdf output i9_2.pdf
   

   If the PDF form to fill in using pdftk is encrypted, you have to decrypt it first. SmallPDF is a good online service, which can help you unlock PDF documents if you don't have the encryption password.

   Notice that some PDF forms (e.g., time card, I-9 form, etc.) cannot be saved (only a blank copy can be saved). after filled in using Adobe Reader. pdftk provides a solution to this problem.

2. Extract pages (from 149 to 186) from the PDF file "training.pdf" as "chap_05.pdf"

   pdftk training.pdf cat 149-186 output chap_05.pdf
   

3. Combine all my i-20s (all PDF files in current directory) into a single file.

   pdftk *.pdf cat output i-20_all.pdf
   

4. Combine scaned pages into the right order.

   pdftk A=20141206171918820.pdf B=20141206171951015.pdf cat A1 B1 A2-3 output i-20.pdf
   

5. Rotate the first PDF page to 90 degrees clockwise

   pdftk in.pdf cat 1east 2-end output out.pdf
   

6. Rotate all pages in the second PDF file to 180 degrees

   pdftk A=m11.pdf B=m25.pdf cat A Bsouth output comed.pdf
   

7. Rotate an entire PDF document to 180 degrees

   pdftk in.pdf cat 1-endsouth output out.pdf
   

References

• Editing PDF Files