Our Project has been using iText 2.17 for a long time. Now we would like to add digital signature to our pdfs and we prefer to use iText7. Can both iText < 5.3 and 5.3+ libraries used together? Will there be a problem in class loader or all the packages different from its predecessor?
Your question starts well, but then gets confusing.
One can't state that you can use versions < 5.3 and versions >= 5.3 together. For instance: you can't use 5.0.0 and 5.5.9 together because they have the same major version number. You will have clashes in package names.
Summarized: all major versions have different package names, so that you can use major versions next to each other without clashes.