公開 and 非公開: release timing and limits on public access
Read 公開 in a future release of historical materials and greetings before a film opens, alongside 非公開 in a private meeting and investigation materials. Preserve the timing and the stated audience restriction.
公開: a future opening and a pre-release event
公開 (こうかい) appears with making materials public and releasing a film. 「町の歴史資料を来月から一般に公開します。」 means “The town's historical materials will be opened to the public next month.” 来月から sets the opening for next month. It does not describe the materials as already available to the public.
「映画の公開を前に、出演者の挨拶が行われました。」 means “The cast greeted the audience before the film's release.” The cast's greetings took place before the film's release. 公開を前に establishes that order; the greetings should not be moved to a point after the release.
非公開: a private meeting or restricted materials
非公開 (ひこうかい) concerns something that is not made public. 「重要な協議のため、会議は非公開で行われました。」 means “The meeting was held in private because it involved important discussions.” The meeting took place in private because of important discussions. Being private does not mean that the meeting was canceled or never happened.
「調査中の資料は一般には非公開です。」 means “Materials under investigation are not open to the public.” The materials are under investigation and are not open to the public. 一般には identifies the public as the group covered by this restriction; the sentence does not specify whether internal staff can view the materials.
Keep time and audience scope separate
For 公開, check both the object and the time expression. Materials scheduled to open next month differ from greetings that already happened before a film's release. A completed greeting does not establish that the release had also taken place, and a future opening should remain a future event.
For 非公開, distinguish the way a meeting was held from the current public-access status of materials. A restriction on the public is not enough to conclude that nobody can access the materials. The sentence supplies a particular audience limit, without explaining every person's access.
Does 「調査中の資料は一般には非公開です。」 mean that nobody can view the materials?
Show the answer and why
No. It says that materials under investigation are not open to the public. It does not establish whether internal staff can view them. Keep the restriction within the scope indicated by 一般には.