.htaccess if-then-like selective rewrite - .htaccess

Question

##### PART 1
# IF:
RewriteRule ^ac$ /ac05182018.pdf [R=301,NC,L]
# THEN:
RewriteCond %{DOCUMENT_ROOT}/downloads/$1 -f
RewriteRule ^(.+\.(pdf))$ downloads/$1 [L]
##### PART 2
# IF:
RewriteRule ^ac64$ /ac64_05182018.pdf [R=301,NC,L]
# THEN:
RewriteRule ^(.*).(pdf)$ download.php?file=$1.$2 [L]
"PART 1" works standalone (= if one requests http...mysite.com/ac then this rewrites to "http...mysite.com/downloads/ac05182018.pdf").
"PART 2" works standalone as well (= if one requests http...mysite.com/ac64 then this rewrites to "http...mysite.com/download.php?file=ac64_05182018.pdf").
I was wondering how to achieve the following:
I need both of them (PART 1 & 2) to be combined, so that either one will be executed, I mean that both work, depending on what file gets requested (either /ac or /ac64).
When I do so though, "PART 1" no longer rewrites to "/downloads/ac05182018.pdf" and then stops executing the rest. Instead it rewrites to "http...mysite.com/download.php?file=downloads/ac05182018.pdf"...
Any idea how to achive this?
Something like:
# IF:
RewriteRule ^ac$ /ac05182018.pdf [R=301,NC,L]
# THEN:
RewriteCond %{DOCUMENT_ROOT}/downloads/$1 -f
RewriteRule ^(.+\.(pdf))$ downloads/$1 [L]
# ELSE
# IF:
RewriteRule ^ac64$ /ac64_05182018.pdf [R=301,NC,L]
# THEN:
RewriteRule ^(.*).(pdf)$ http...mysite.com/download.php?file=$1.$2 [L]
Maybe with RewriteRule flag [S] or
<If ... >
...
</If>
<Else>
...
</Else>
as both seen in the answers of How to write "if...else" mod_rewrite statements in .htaccess ?

Answer 1

First this line RewriteCond %{DOCUMENT_ROOT}/downloads/$1 -f will prevent next rule RewriteRule ^(.+\.(?:jpe?g|pdf))$ downloads/$1 [L] to be executed because you don't have downloads yet in document root .
After remove it , there will be two rules to .pdf files :
RewriteRule ^(.+\.(?:jpe?g|pdf))$ downloads/$1 [L]
and this :
RewriteRule ^(.*).(rar|zip|pdf)$ http...mysite.com/download.php?file=$1.$2 [R,L]
Unless you do some conditions one of them both will match .pdf and that will give error .
Moreover , don't forget to to put L flag like here :
RewriteRule ^ac64$ /ac64_05182018.pdf [R=301,NC]
it should be RewriteRule ^ac64$ /ac64_05182018.pdf [R=301,NC,L]
L flag means if the rule matches, no further rules will be processed.
If i understand you your code should be like this :
RewriteEngine On
#Part 1
RewriteRule ^ac$ /ac05182018.pdf [R=302,NC,L]
RewriteCond %{REQUEST_URI} !(downloads|ac64)
RewriteRule ^(.+)\.(jpe?g|pdf)$ /downloads%{REQUEST_URI} [L]
#part 2
RewriteRule ^ac64$ /ac64_05182018.pdf [R=302,NC,L]
RewriteCond %{REQUEST_URI} !downloads
RewriteRule ^(.+)\.(rar|zip|pdf)$ /download.php?file=$1.$2 [L]
Clear browser cache and test it , if it is ok , change any 302 to 301 to get permanent redirection .
Note: any .pdf files not start with ac64 will be treated by the ac rule , first rule .