Convert PDF to images to PDF

Imagemagick rasterizes everything. You could simply do:

convert -density 200 input.pdf output.pdf

Of course the recipient might decide to run an OCR program on your file. The density parameter sets the resolution expressed as PPI.

PDF to Image

The poppler-utils packages includes the pdftoppm utility, capable of converting PDF files to either ppm, png or jpeg format:

pdftoppm -png file.pdf prefix

This will make files name prefix-X.extension where X is the page number (each file outputted will be one page of the PDF converted) and where extension is whatever output type you choose.

By default the resolution is 150dpi. You can increase the resolution (for higher quality output) by using this command:

pdftoppm -rx 300 -ry 300 -extension file.pdf prefix

And to print only one page, do:

pdftoppm -f N -singlefile -extension file.pdf prefix

where N is the page number, starting with 1.

This method is a lot faster and less clunky than using the imagemagick package as mentioned in other posts. Although you do have to use it to convert back.

Image to PDF

This requires the installation of the imagemagick package. To do this, do:

sudo apt-get install imagemagick

The imagemagick package has a utility called convert which will do just as it is named; convert. To make use of it in the way that you want, run it like so:

convert file.extension file.pdf

This will make a PDF of only that single page, though. To combine all outputs of the previous command for converting to images, use this command:

convert *.extension file.pdf

This will grab all files in the directory that you are in with the extension extension and convert them into a PDF file named file.pdf.

Note

I chose to format my answer in the way of two separate commands as to give the OP flexibility and understanding of the task that they are trying to complete, instead of one command linking the two. Of course, if that solution is better for them, then I encourage them to upvote/mark as answered that particular answer.

I always use pdftoppm to convert pdf to png images. simply execute the following command

pdftoppm -f [startpage] -l [end page] -r [resolution (use 300 or 600) -png <pdf file location> [Prefix for your image files]

and then execute

convert prefix-[number].png file.pdf

but there are programs that convert these images back into text. making your pdf uncopyable is very hard. You could add an encryption protection to prevent copying and printing which will make is (slightly) more difficult to copy and print your pdf file