Publishing scans without being identified is a tough proposition. There are multiple risks of information leak, and mitigation is technically complex. However, anyone determined to do so can learn the appropriate techniques, and there is free software to accomplish the task.
Disclaimer: Although I consider myself technically knowledgeable about the mentioned issues and I've included references where they exist, some parts of this answer are speculative.
Do scanners add any visual unique fingerprint (or even worse:
information about the connected device etc.) to every scanned page?
This seems likely, considering that some printers do so. There isn't much information available on scanners, though.
Do scanners add any digital (e.g. binary) fingerprint (or even worse:
information about the connected device etc.) to every scanned file?
If you're doing a scan from an attached PC (as your question implies), the answer is no, the scanner can't. Scanners attached to a PC transfer raster image data, not files, so it can't possibly add data to a file it doesn't have access to.
However, you should consider that a digital fingerprint could be added on the scanning software of the PC.
Also, if the scanner is standalone (it saves files to a USB drive, or sends them by email), this is a definite possibility.
Do scanners have a unique 'technical unavoidable' fingerprint, so every scanner scans differently? And is this fingerprint computable or even stored somewhere?
Or does the 'institution' that wants to deanonymize me have to have access to my scanner to make an comparison?
Yes. Most modern scanners use CCD sensors, which are uniquely identifiable by their noise pattern, using specialized software.
Other plausible visual fingerprinting targets:
Using these kind of fingerprinting techniques, it seems likely that the scanner model and paper type can be identified from the scans, but identifying the specific scanner and paper page used would be hard (perhaps impossible) without access to them for comparison purposes.
Do PDFs 'store' any information related to the host computer in them?
Yes, there's even a NSA article about it. While dealing with scanned documents, you'll need to be aware of image file metadata, which can also be present on PNG and JPG files, for example.
Another risk that you didn't mention is that the scanner itself may store a copy of your scan. Big printers do
Of course, this isn't a exhaustive list of risks - merely what has come to my mind in the couple of minutes it has taken me to write this answer. I'm pretty sure researchers, intelligence agencies and police paid to do so can come up with better ideas!
The easiest, safest and obvious mitigations are don't use a scanner that can be tied to your identity, and destroy the scanner after the fact. Of course, this is not always attainable, so what else can you do to protect yourself?
Don't use a stand-alone scanner - especially a networked one. If you really must, convert its output to a pure image without metadata.
For (at least partially) mitigating fingerprints added by software, you'll want to use open source software, both for the OS and the scanning program.. Avoid using your personal PC for scanning, or at least, use a secure live OS
For detecting deliberate visual fingerprinting, the best option would be to scan a blank page and look for obvious anomalies. These might be very small, so you may want to use a image editor to crank up the contrast.
For sensor, paper and visual fingerprinting in general, you want to destroy subtle scanning artifacts. Use a image editor to:
- Add noise
- Use a noise reduction filter (with aggressive reduction)
- Distort the image (by applying multiple camera "lens correction", for example)
- Convert the image to grayscale
- increase the contrast (or, preferably, completely convert to black-and-white)
- Reduce resolution (preferably by a near-to-irrational factor)
- Compress the image (high JPEG compression, for example)
In general, do everything you can to obfuscate and reduce the amount of information contained in the image while keeping the document reasonably readable.
Finally, after all the other steps, remove the medatadata from your files. You can use specialized software to do this.