I recommend you to investigate a bit different approach for generating PDF from HTML. Instead of trying to find a perfect HTML to PDF library for your technology stack (e.g. PHP) try to take advantage of using Headless Chrome for PDF generation. We used this approach to implementing pdf generation for this free resume builder.
For example, try to make use ...
You can open PHP files in VS, and I expect the comparison function will still work.
However, and generic text comparison approach is to use Notepad++ with the Compare plugin:
If you want a PHP IDE, Eclipse will allow you to compare files too (Compare With Other Resource function):
Are you certain that you want it to be online?
The problem that I foresee with that is that it might be ok for single pages, but PHP apps tend to chain from page to page. E.g. show login page, on click submit button, go to validation page, from which go to either error page & page to login, or to main app page (from where, possibly go to other pages).
Visual Studio Code is a free open source IDE available on Windows, macOS and Linux (since you didn't mention your OS). Just add a PHP extension for the syntax. MySQL extensions are also available if you want. Otherwise, you also have Visual Studio Community, which is free.
The easiest way would be opening the user's email client and having them email the note content to their self. This would ultimately come down to one line of code:
location.href = 'mailto:email@example.com?subject=my_subject&body=body_content'
Since you seem willing to get your hands dirty with some programming, you can set up a ...