Or do they do a sort of selective search? If I apply, is there something I have to do in advance to ensure that will my file be opened, at all?
In my department, postdoc applications are read first by the faculty in the applicant's research area. Each area chooses about five of its top applicants, and those chosen applications, from all areas, are then read by the department's executive committee, which decides what offers to make.
Applications for tenured and tenure-track positions are read by members of the department's personnel committee --- not all applications by all members. The best applications are then brought to the attention of the whole personnel committee. Also, faculty members who are not on the committee can and do read applications and bring particularly strong ones to the committee's attention.
So the answer to the question, as clarified in the comment "will they open the pdf?", is that all the applications get looked at by faculty who provide input into the search process, though not necessarily by the members of the search committee.
There are job markets where the first challenge for an applicant is that the recruiter even looks at their application (which leads to several recommendations how to achieve this, most of which seem bizarre). The reason for this is that there are so many applicants for each position that it is unfeasible to even have a one-minute look at each application. This mostly happens in big job markets that are filled with candidates.
Now, while some academic job markets are filled with candidates, they are certainly not big. Academic jobs are very specialised and there are often only a few people worldwide who have a reasonable chance of getting the job. Therefore, I see no reason why your application would not at least be looked at.
Of course, your application may be discarded very quickly due to not fulfilling some crucial criteria, or you may not be seriously considered for the job because this position is tailored to a known person and the job posting is purely pro forma.
Concrete search procedures vary, but (for faculty searches) often follow along these lines:
- An administrative assistant does a pre-filtering, and discards all applications of people not meeting the formal requirements (e.g., no PhD).
- One or two people in the search committee (or, sometimes, an external person such as a postdoc) do an initial ranking. Essentially, they compile some basic info on each applicant, such as which school the candidate graduated from, which field they are in, how much and where they have published, and bibliometrics such as the h-index (if the school cares about such things).
- Based on the list, an inofficial shorter list is created of candidates that warrant a closer look. For these, some or all members of the committee read the entire application package (to varying degrees of detail), and discuss. Potentially, all or a subset of candidates on this list are sent out to external review. The committee decides on an official short list.
- Candidates on the official short list are interviewed and ranked.
At some point during this process recommendation letters may be collected.
Note that I am not claiming that this is exactly the process that every university uses, but these basic steps I have seen in three different universities in Europe. For postdocs, the search procedure may be much more informal.
Do postdoc/professor search committees read the files from all candidates?
Not necessarily. For a call that receives many applications (and this realistically includes most calls), there is usually some amount of pre-filtering before applications are studied in much detail.
EDIT: I guess it also depends on what you mean with "read". I agree with Andreas Blass that it is highly likely that some member of the search committee will at some point have opened each PDF (at least of the candidates that are not filtered out due to not fulfilling basic job requirements), but I would not count on each application package actually being read in any detail by at least one professor.