There are many different ways of changing documents. With the current technology it is possible to change even pdf files without it being visible.
Look in any case for different fonts or sizes of text in a document. Any unusual difference might be a sign that the original document has been altered.
Also look for the name and number of the Notified Body (number is 4 digits). To be sure the Notified Body is indeed genuine and notified/competent for PPE, you can check on the Nando database (https://ec.europa.eu/growth/tools-databases/nando/index.cfm - on the site it is possible to search by number, by name of the notified body, by country, by legislation 2016/425 for PPE).
If you have any doubts, do not hesitate to contact the organisation of PPE suppliers in your country, the market surveillance authorities in your country and/or the Notified Body mentioned on the document (some NBs have a website where you can check validity of certificates) to check the validity of the document.