This is my .htaccess file in the head
<FilesMatch "\.pdf$">
RewriteRule ([^/]+)\.pdf $ - [E=FILENAME:$1]
<If "%{REQUEST_URI} =~ m#^/wp-content/themes/my_theme/pdf/fr/.*#">
Header add Link '<https://www.example.com/wp-content/my_theme/theme_SES/pdf/fr/%{FILENAME}e>; rel="canonical"'
</If>
</FilesMatch>
And I'm receiving the following error in the error.log:
RewriteRule: bad flag delimiters
What's wrong with the .htaccess file?
RewriteRule ([^/]+)\.pdf $ - [E=FILENAME:$1] ------------------------^
You have an erroneous space before the $. So the - is being seen as the flags (3rd) argument, hence the error.
It should be written like this:
RewriteRule ([^/]+)\.pdf$ - [E=FILENAME:$1]
Aside: You should be OK in this instance, but note the following warning from the docs:
Although rewrite rules are syntactically permitted in
<Location>and<Files>sections (including their regular expression counterparts), this should never be necessary and is unsupported. A likely feature to break in these contexts is relative substitutions.
For example, your code block could be rewritten more simply as:
RewriteRule ^wp-content/themes/my_theme/pdf/fr/(?:.+/)?([^/]+)\.pdf$ - [E=FILENAME:$1]
Header add Link '<https://www.example.com/wp-content/my_theme/theme_SES/pdf/fr/%{FILENAME}e>; rel="canonical"' env=FILENAME
This could perhaps be simplified further knowing your actual URL structure.
And, depending on your other directives, you may need to change FILENAME in the Header directive to REDIRECT_FILENAME. (Depending on whether there is a loop by the rewrite engine - which there usually is if the standard WordPress directives are being used.)
Note the addition of the env=FILENAME argument on the Header directive, which sets this header conditionally on whether the FILENAME env var is set.