My understanding is - and my internet-research reassured me - that one uses lower-case letters for the probability density function and upper-case letters for absolute probabilities. 'p' vs 'Pr' for example.
Now, I'm reading an established textbook ("Computer Vision: Models, Learning, and Inference" the PDF-textfile is available here: http://www.computervisionmodels.com/) and the author uses 'Pr' to notate probability density functions. See bottom of page 40 for example, where one can find the following formula:
Hence, I'm wondering whether this notation is common. Because I'm thinking about adapting it.
You are right. It is very common to denote absolute probabilities with uppercase letters and density functions with lower case letters. The letters "P" and "p" are the most commonly used but, of course, you can find other letters as well. In the textbook you are referring to they are just using a less standard notation. This is also very common.