When I open the file, I get a message that there was an error opening this document. The file is damaged and cannot be recovered. A PDF file, like any other file, can be damaged. Damage to pdf files is quite rare, and many may never even encounter this problem, but we will still consider how to recover a pdf file further. Since damaged pdf files are quite rare, there are not so many programs for recovering damaged pdf files. The most common reason for such errors is the incorrect operation of various software: browsers, mail programs, operating system, antivirus, firewalls and others.
There are a lot of services offering to magically solve all your problems with opening corrupted .pdf files. Just ask Google and it will readily show you a couple of dozen relevant links. Further - the choice is for the user. It all depends on the capabilities of your wallet, as well as scrupulousness in matters of data privacy and Internet security. Errors appear due to damage to the hard disk, incorrect operation of the antivirus, browser or mail client. There are dozens of possible reasons, but if this does not happen every day, there is no reason for concern - just restore the damaged document and forget about it.