: If you are looking for specific fan-made expansions or mods, ModDB is the industry standard for safe, community-vetted files.

You should be with this specific file. A file named DOWNLOAD FILE – HALF-LIFE: OPPOSING FORCE.RAR —especially with the broken character encoding ( – )—is a classic hallmark of malware or a "browser hijacker" scam .

Genuine game files or reputable mods are almost never named with "DOWNLOAD FILE" as part of the filename. This naming convention is typically used by automated scripts on suspicious websites to trick users into downloading malicious content. Why this is likely a threat:

: The official Steam store page is the safest way to get the game. It is frequently on sale for a very low price.

If you want to play the game safely, I recommend using these official or highly-trusted sources:

: The "–" in the name suggests it was generated by a bot or a site using mismatched character sets (often associated with low-quality or malicious Russian-hosted "warez" sites).