I’m not sure this true - PDF is an open standard. The issue isn’t with layout and reproducibility - a good PDF maker and a good reader will give you an accurate representation of how it looks on all devices.

There isn’t a dedicated FOSS tool for make PDFs; Libre Office and Inkscape do a decent job. But they’re not dedicated PDF makers and the real problem is building fillable forms and signatures.

But there is a proprietary alternative called Master PDF that is a dedicated and supports all the PDF standard features I believe; one perpetual license is $80 compared to Adobe subscription based charging. I’m not aware of other options myself but they may exist.