OK well, I did some more research and although tohuwawohu's method does work, I found it easier to use a program called pdftoppm to achieve what I wanted done. Since I am pretty much a layperson when it comes to using command line apps, I will do my best to explain how I got this to work for me.

1. Navigate to the folder containing the .pdf you wish to edit and open a terminal there. I did this by using the sample command:

   
   
   

   cd ~/Documents/PDF
   
   




2. Let's say the file I want to edit is called Sample.pdf What I want to do is use pdftoppm to create image files of each page of the .pdf. Several formats can be chosen (see the man pages link above) but I prefer to use .png. The basic command looks like this:

   
   
   

   pdftoppm -FORMAT FILENAME.pdf PREFIX
   
   

   
   
   

   or in the example above:

   
   
   

   pdftoppm -png Sample.pdf Sample
   
   

   
   
   

   This command creates an image file of each page in the same folder as the original .pdf file with names like Sample-01.png, Sample-02.png and so on. I have tried it with the .png and .jpeg extensions successfully. .jpg is apparently not supported.




3. Then I just use Archive Manager by selecting all the newly-created image files, right-clicking, and choosing "Compress" from the context menu. I then choose the archive format I prefer (in this case .cbz or Comic Book Zip) and create the new archive.




4. Now I have a shiny new .cbz file called Sample.cbz which I can then view with my Comix reader!

Hopefully what I have posted above makes enough sense that someone else can learn from it. If I need to change it in any way please let me know.