PDF looks good on computer, printout is full of black boxes

[Mike5EFA]

We're storing scanned PDFs in a web archive where we can view and print them. Many of the PDFs are scanned handwritten forms or different sorts. When viewed in browser, the PDFs look good. But when printed, some PDFs get distorted with black boxes as illustrated in the attached image.

It is difficult to go into details about our technical environment, PDF versions etc but I'd still like to ask the community if this behavior is familiar to some and/or if there might be known root causes for this.