Do postdoc/professor search committees read the files from all candidates? 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.