Develop a PDF failproof and hacking-proof "marking" software to be run on Linux machine

Hello everybody, GOAL I have on my forum a database of files downloadable and uploadable by end users. I need a software application to “mark” every single file we have on our database with a specific and unique encrypted sequence, totally invisible to end user. This sequence has then to be saved on a database and associated with the file itself. I will later use this information to check, when my users will upload new documents, whether these already are on our database or not. SOFTWARE SPECIFICATIONS • The software will run on a Linux machine • The “mark” has not to affect the user experience in reading the pdf in any way • I don’t need a DRM application. Files have to be fully open-able with no “expiration” date. I just need to mark each file “invisibly”. • The software has to operate so that for those who want to hack or circumvent this encryption it is impossible to: 1) delete it with other PDF watermarking/unlocking softwares 2)reverse engineer the algorithm • The software application has to be built under the “black box” principle: I provide some input, it provides us back with the output. These are the functions that we need 1)Checkfile 2)ProtectFile 1) Input: file_name; file_path Output: Outcome (Y|N); steg_unique_sequence_found [if any]; duplicate_file_found [if any]; duplicate_file_path [if any]; 2) Input: file_name; file_path Output: Outcome (Y|N); Error [if any]; Outcome_file_path; outcome_file_name; outcome_file_specific sequence; steg_unique_sequence • Encripted sequence has to be added to every single page • The software has to be “fail proof” and work on any type of pdf (even protected ones), with pages of any dimension). OTHER • I need the property of the source code of this application. • I need a ready-to-use software with a proper instruction manual • I need a developer with previous experience in programming and encryption algorithm