How to write small size images to a single pdf files

Hi I have a cell array that recursively holds images i-e Say a loop runs for 20 iteration for each iteration it will store a different 400X400 image. I want to write all the images to a single pdf file.

if true
  ImagesTowWrite{};
  for a=1:20
     ImagesToWrite{a}=I;
  end
%i want to do save all images to a single pdf file now ?
end
Thanks anyways