First of all; I’m not sure if you are allowed to make modifications to that font. So please read the license agreement (within the font), or even better contact the font vendor.
Maybe there is an issue with hinting or with the OpenType layout features. Can you provide more information, so we can better guess what goes wrong?
Free alternatives[edit]
Arial is a proprietary typeface[28] to which Monotype Imaging owns all rights, including copyright, design and trademark rights.[29] Its licensing terms prohibit derivative works and free redistribution.[30][31][32][33][34][35][36]
There are some free software metric-compatible fonts used as free Arial alternatives or used for Arial font substitution:
Liberation Sans is a metrically equivalent font to Arial developed by Ascender Corp. and published by Red Hat in 2007, initially under the GPL license with some exceptions.[37] Versions 2.00.0 onwards are published under SIL Open Font License.[38] It is used in some GNU/Linux distributions as default font replacement for Arial.[39] Liberation Sans Narrow is a metrically equivalent font to Arial Narrow contributed to Liberation fonts by Oracle in 2010.[40]
URW++ produced a version of Helvetica called Nimbus Sans L in 1987, and it was eventually released under the GPL and AFPL (as Type 1 font for Ghostscript) in 1996.[41][42][43] It is one of the Ghostscript fonts, a free alternatives to 35 basic PostScript fonts (which include Helvetica). It is available in major free and open source operating systems. Nimbus Sans L is also used in some software for Arial font substitution.
FreeSans, a free font descending from URW++ Nimbus Sans L, which in turn descends from Helvetica.[28][44] It is one of free fonts developed in GNU FreeFont project, first published in 2002. It is used in some free software as Arial replacement or for Arial font substitution.
Thanks for the feedback, I do understand the legal issues and I am doing the experiments the way I am to understand the technical side of it first.
The attached file shows a portion of the screen capture with the top line from the unmodified font files and the bottom line from the modified files. For both cases the software drawing the text is completely unmodified.
The modified files (arial.ttf, arialbd.ttf, arialbi.ttf and ariali.ttf) have only been changed by loading them into FC and exporting them to to a new folder with the same names. I am not changing anything within the font and have not changed any settings from the default.