greyed out files across apple hardware

One of the benefits of apple products is when everything works across platforms like iCloud.

One consistent problem I have had over the last years that no one seems to answer on the apple side is why files that I can see saved in apps on my iPhone are greyed out while others can be opened without rhyme or reason that any one can explain, and apparently without solution. Often two identical documents have been saved in the same format or program, with one greyed out and one not.

Is there any resource that can give a foolproof approach to saving files in multiple formats so that the chances of being able to open them on an iPhone or iPad are maximised?

Thanks