Convert PDF to images to PDF
Solution 1:
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.
Solution 2:
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.