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Off the top of my head, I believe that the expression should be

RewriteRule ^/wp-content/uploads/2022/03/Terminanfrage.pdf$ https://otherdomain.de[R=301,L,NC]

Note the forward slash after the caret. The ^ character in a POSIX RegEx (the kind of RegEx Apache uses) matches the starting position of the string. RewriteRule operates on the URL path which always includes the leading slash in Apache as per Apache's manual:

In per-directory context (Directory and .htaccess), the Pattern is matched against only a partial path, for example a request of "/app1/index.html" may result in comparison against "app1/index.html" or "index.html" depending on where the RewriteRule is defined.

The directory path where the rule is defined is stripped from the currently mapped filesystem path before comparison (up to and including a trailing slash). The net result of this per-directory prefix stripping is that rules in this context only match against the portion of the currently mapped filesystem path "below" where the rule is defined.

Directives such as DocumentRoot and Alias, or even the result of previous RewriteRule substitutions, determine the currently mapped filesystem path.

This is impossible to comprehend as it's written. Basically, your virtual host maps a filesystem path, let's say /home/myuser/www, to the domain name www.example.com. Therefore the URL https://www.example.com/foo/bar is internally represented as something like VirtualHostRoot/foo/bar. For RewriteRule purposes in a .htaccess file placed in the web root (i.e. what WordPress has), Apache throws away the VirtualHostRoot part and we have /foo/bar with a leading slash.