Error on OCR

We start to have some strange OCR error on a process that have being running for more than a year. The robot reads multiple .pdf, but it start to read the first one correct, the second one with some errors an on the 3 or 4 one it starts to read a very different part of the document.
We use Google OCR and we tried to change it to another OCR but we got the same error.
Is it a knowing problem?
We need help.
Thank you