TCPDF is a PHP library that was first released in 2006, and is still used today in many projects for generating PDFs such as invoices and reports.
One detail that is easy to overlook is that it includes its own HTML and CSS rendering logic. Rather than relying on a browser engine, TCPDF parses a limited subset of HTML and CSS directly in PHP. This allows developers to define layouts using familiar markup, but it also means the rendering behavior can differ from standard web browsers.
The library also includes built-in support for features such as barcodes, QR codes, and digital signatures, without requiring additional dependencies.
A downside of this approach is that its HTML/CSS support is limited and sometimes inconsistent, especially for more modern layouts. Complex designs that work in a browser may require adjustments or manual positioning when rendered through TCPDF.
Overall, it functions not just as a PDF generator, but as a self-contained document rendering system implemented entirely in PHP.