Uh... that's strange.
I did this in a quick test installation:
Page.php

static $has_one = array(
    'PDF'  => 'File'
);

function getCMSFields() {
    $fields = parent::getCMSFields();
    $fields->addFieldToTab("Root.Content.Files", new FileIFrameField('PDF'));

     return $fields;
}

<div id="PcFile">
    <a href="$PDF.Link">$PDF.Title</a>
</div>

Everything worked as expected, i was able to upload a .pdf and there was the correct download link showing up on the frontpage (with neat little pdf icon)

So, i am a little bit lost with your problem, sorry.
Maybe some .htaccess issues?

Cheers,
Christian

Hi Invader,

thanks for your efforts!

I left the .htaccess files untouched. So the .htaccess in my assets folder contains:

Deny from all
<FilesMatch "\.(html|htm|xhtml|js|css|bmp|png|gif|jpg|jpeg|ico|pcx|tif|tiff|au|mid|midi|mpa|mp3|ogg|m4a|ra|wma|wav|cda|avi|mpg|mpeg|asf|wmv|m4v|mov|mkv|mp4|swf|flv|ram|rm|doc|docx|txt|rtf|xls|xlsx|pages|ppt|pptx|pps|csv|cab|arj|tar|zip|zipx|sit|sitx|gz|tgz|bz2|ace|arc|pkg|dmg|hqx|jar|xml|pdf)$">
Allow from all
</FilesMatch>

This is my top level .htaccess:

### SILVERSTRIPE START ###
<Files *.ss>
Order deny,allow
Deny from all
Allow from 127.0.0.1
</Files>

<Files web.config>
Order deny,allow
Deny from all
</Files>

ErrorDocument 404 /assets/error-404.html
ErrorDocument 500 /assets/error-500.html

<IfModule mod_alias.c>
RedirectMatch 403 /silverstripe-cache(/|$)
</IfModule>

<IfModule mod_rewrite.c>
SetEnv HTTP_MOD_REWRITE On
RewriteEngine On
RewriteBase /

RewriteCond %{REQUEST_URI} ^(.*)$
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule .* sapphire/main.php?url=%1&%{QUERY_STRING} [L]
</IfModule>
### SILVERSTRIPE END ###

I've made good progress in my ss efforts today ... If I get this pdf thing fixed somehow, I will be very happy!

Ah, ja...
This is my patched (more info here:http://silverstripe.org/general-questions/show/15505 ) .htaccess file from /assets directory:

Deny from all
<FilesMatch "\.(html|htm|xhtml|js|css|bmp|png|gif|jpg|jpeg|ico|pcx|tif|tiff|au|mid|midi|mpa|mp3|ogg|m4a|ra|wma|wav|cda|avi|mpg|mpeg|asf|wmv|m4v|mov|mkv|mp4|swf|flv|ram|rm|doc|docx|txt|rtf|xls|xlsx|pages|ppt|pptx|pps|csv|cab|arj|tar|zip|zipx|sit|sitx|gz|tgz|bz2|ace|arc|pkg|dmg|hqx|jar|xml|pdf)$">
	Allow from all
</FilesMatch>

# We disable PHP via several methods
# Replace the handler with the default plaintext handler
AddHandler default-handler php phtml php3 php4 php5 inc

<IfModule mod_php5.c>
	# Turn the PHP engine off
	php_flag engine off
</IfModule>

It's pasted here again.
Replace yours and give it another try :-)

Hallo Christian,

und danke... :) But that wasn't it. Same error.
The path in the database is fine, there seems to be a problem with instantiation a file object.

This snippet from File.php just delivers "/assets".

function Link($action = null) {
return Director::baseURL() . $this->RelativeLink($action);
}

function RelativeLink($action = null){
return $this->Filename;
}

Hey,

something really weird happend. I was just working on my templates when I suddenly recognized a file icon on my detail page. I wanted to find out what the matter was before, checked on special characters etc., but ss has no problems with special characters.

So I have my longed for link, but I can't say what the problem was.

Hehe,
M A G I C

... or a flushed cache or something.