Docx files are editable text file whereas PDF files are not editable easily. So if you want to protect your finalised Docx file from unintentional changes then it is a good idea to covert your final docx file to pdf for printing or just to sent it to someone.
Pdf files are printed better and are easier to handle than Docx files, so PDFs are mostly used for printing or sending files to someone online or through a usb. Keep in mind that if you want to keep or send the editable file then docx file format is a better option as it can be edited by anyone.
So, there are two ways to convert your docx document file to PDF
MS Word, Apple Pages and most other text editing softwares have the option to save docx files as PDF. So, to if you have MS Word or anyother docx reader installed then simply open the docx file using it and Click on File/"Save As" feature.
You will be prompted to a dialogue box where you can speciify a location where you want to save the file and Just above the Save button you will be asked to select a File Format to save the file. By default this format will be docx, so you have to change it to PDF in the drop down. Once pdf file format is selected, simply save the file and it will be converted to PDF from Docx and saved in the selected destination.
If you don't have Microsoft Word or any other docx software installed then you can convert the docx file to pdf using Docx to pdf converts online. Just search "Docx to PDF Online" on Google and pick any of the sites where you can upload the docx document and it will be converted to PDF.