[Marinolino]

In Windows I'd use pdf editors like Adobe Acrobat, PDFelement or Abbyy Finereader.

So, maybe you could try some free iOS pdf editor like PDFelement.

I am talking about page-size tool that usually can be found together with the crop tool or page-boxes.

There is a choice to choose from the fixed page sizes like Letter, Legal, A4 etc. or to use a custom tailor mode for a more precise page width & height.

I also check the "center" checkbox to center the text in the middle of the page.