Is a document considered falsified just because it has no stamp or signature?
A lack of a signature or stamp could be one piece of evidence suggesting that a document is falsified or fabricated, but there is no universal requirement that a document have a signature or stamp, and even if one is missing there are a variety of legitimate reasons (e.g. printing error or careless oversight) that this could have happened. A hard to read seal or signature is even more common in legitimate cases due to low quality copying or a low quality printer. Printers and copiers are notoriously error prone as anyone who has worked in an office environment knows.
Often a document is distributed in proof form, unsigned and sealed to the recipient, in order to allow proofreading of name spelling, etc., who may actually have earned the executed one but accidentally submit the unexecuted version to an employer.
A document is only fabricated if the facts that it attests to are not true and it was made knowing that they were not true.
Usually, a decent document fabricator will also include a fake signature or seal, so usually a lack of a signature or seal is an indication of either non-fabrication or an extremely amateur job of fabrication.
If you want to know if a document is fabricated, the surefire way to get to the truth is to contact the issuer of the document to determine if it is real thing.
From a legal perspective, the main effect of a lack of a signature or seal is that the document is likely to not be self-authenticating and may require more extrinsic proof to show that what it purports to demonstrate is really true.
Can the person be accused of falsifying documents, sued and so on?
If the document is actually false and was knowingly submitted as a false document, then the person who submitted the fabricated document has attempted to defraud the company and is not immunized from liability for that, necessarily, by the lack of a seal or signature.
Of course, anyone can be accused of falsifying documents whether it is true or not.
Fraud in an application for employment is frequently a good cause ground for terminating someone's employment even if they could otherwise only be terminated for cause and not just at will. Indeed, when incidents of application fraud surface, such as a false statement made on a resume, the employee routinely is fired no matter who competent their performance on the job was.
For example, I recall a case of a university president who was fired for claiming to have a degree that wasn't actually earned, despite serving more or less competently.
To sue someone for fabricating a document the party to whom it was provided would have to show that the fraud caused them economic harm, which is something that would frequently be difficult or impossible to prove. And, in the absence of proof of harm, the question of whether the employer was justified in relying upon the fabricated document because it had no stamp or signature would never come up.
Ultimately the question of whether their reliance was justified would be a question of fact to be decided on a case by case basis, but for the reasons set forth in the first part of this answer, it is likely that the lack of a seal or signature would not make their reliance on the fabricated document, which was intended by the applicant, unjustified.