You can convert a multipage PDF into multiple PNG files with a simple Bash statement:
convert -density 300 your.pdf -quality 100 -scale 825x1125 your-%d.png
Now you can change the images with a simple GFX application like Kolourpaint. You can also create new images with the same size and reorder your images by the number within the filename.
After you have finished your work, recombine the PNGs into a PDF:
convert your-*.png your-new.pdf