Open Source tool/library to modify the size of an element in .pdf files?

Can anyone suggest an Open Source tool or library to alter the size of a field contained in a .pdf file? I used libraries to print barcodes in the past, though never had to modify existing ones. My doctor sends me prescription sheets in .pdf in which the barcodes (code39) don't use all the space available and once printed are so small that all barcode readers at the chemists fail, forcing them to to enter all fields by hand, thus creating a line behind me. By looking into the files I saw they're created using "Foxit Quick PDF Library 17.11", and importing them into Libreoffice I can easily find all barcode fields and expand them, therefore I wonder if I could automate the operation by writing a function that reads the file structure, finds all fields containing barcodes (assuming they're labeled somehow), applies the necessary changes then saves the modified files.