First: there are two of them
The names are nearly identical and the products are not. zplpdf.com.br is the Brazilian ZPL2PDF, with marketplace presets and a subscription in reais; zplpdf.com is this one, international, four interface languages, priced in dollars.
If you came looking for the Brazilian one, that comparison is at /vs/zplpdf.
What ZPLPDF gets right
It is the closest implementation to ours anywhere: conversion runs in the visitor's own browser, and the site states that ZPL codes are not stored on their servers. On the question that matters most for a document carrying a customer's name and address, the two of us are on the same side.
It also ships syntax validation, PNG and JPEG export, multi-file upload and an interface in four languages, which is a wider surface than most free converters manage.
Where the two diverge
At the counter. Rendering in the browser costs the operator nothing per label, and yet ZPLPDF's free tier is metered: ten PDFs a month, at most 75 labels in each. Anyone shipping daily is past that in the first week.
The arithmetic here is different twice over. It is daily rather than monthly, and it is far bigger: the free plan converts dozens of labels a day against ten PDFs a month. And it is done in your own browser rather than on a server of ours — there is no account tying the count to you, and neither file size nor batch size enters into it, since a whole ZIP counts as the labels inside it.
When ZPLPDF is the right answer
If you convert a handful of times a month and want validation and editing tools in the same place, their free tier covers it at no cost.
And if your team works in Spanish or Chinese, their interface covers languages ours does not.