We have a finished magazine which was designed to A4 paper size but I need to have printed in the U.S. The magazine is in Adobe PDF. Conversion distorts the aspect ratio of all the pages. How can I get the A4 designed magazine printed to letter size standards without having to re-design the entire mag.
"Printed on the US" does not mean that you can not print a magazine of other sizes.
You can print a magazine of whatever size you want, for example a mega Magazine of 50x70 cm. or a tiny one of 10x10cm.
There are a lot of "standard" papers, for example 61x90cm or 70x95cm.
Your a4 magazine can easily be printed in thoose papers.
So you can print that in US or almost any part of the world.
The question is, is A4 a terrible size for your magazine? For example, the case that you have a long tradition of a magazine Letter size? or the A4 size can give you an "European look" that is interesting?
If you need really to convert the magazine you need the source files and reformat manually.
For quality output, there's no way around a rework of the piece. You don't need to "redesign" the entire thing, but it will require manual adjustment of the layouts.
There's no shortcut if you want good output for both sizes.
I don't know if you're thinking about reflowing text and everything? In that case you should try to get a hold in the original layout document.
A hotfix could be a combination of resizing and adding equal margins on the sides, and maybe packing things a little bit more in the vertical direction. Might turn out fairly ok.