How do I convert a document file into pdf in php?

You can use TCPDF class, it helps you to generate PDF documents from PHP script.
Take a look here: http://www.tcpdf.org/

I guess - You can create it using COM class.
Following would be the steps to do it: - Read .doc file using COM class http://www.php.net/manual/en/class.com.php - Write .doc content into new pdf file using your favorite PDF library.
- Rename and Save newly created file.
Important: COM class only works with Windows.

I use online to convert to pdf it really useful

I suggest to use PDF library like (TCPDF) to create PDF with arabic language .