Printing Text onto uploaded PDFs via Webform

For my customers I need a special webform to let my customers upload PDFs. The script should print some adress that customer enters into form onto PDF at certain coordinates. Steps for customers: 1. Customer uploads pdf (fixed size) 2. Customer fills out the following fields: * Sender Adress * Sender Adress 2 * Sender Street * Sender Housenumber * Sender ZIP Code * Sender City Name * Sender Country is fixed * Receiver Adress * Receiver Adress 2 * Receiver Street * Receiver Housenumber * Receiver ZIP Code * Receiver City Name * Receiver Country is fixed 3. Customer clicks on 'Submit' and customer gets PDF with address printed on it. The size of the uploaded document is always the same (DIN A4 size). The coordinated of the above fields should be flexible for us to change. Please use documentation, so that another programmer is able to read the code and change it easily.