I am very interested in how this is done.
Before 'View Extras' was introduced in MC22 the way to view PDF booklets was to create a custom Link.
This post
https://yabb.jriver.com/interact/index.php?topic=73415.msg497686#msg497686 describes a way of doing this (please read this first).
If you look at the screenshot in my original post you will see a Link called Booklet (next to the Amazon Link).
I had some issues with the original Link information, it caused MC to have a very long unresponsive startup due to the use of the IsMissing() expression, I personally don't like 'anonymous' file assets like 'booklet.pdf' or 'folder.jpg' and the Link as published still started an external PDF viewer.
I'll describe the changes I made to the Link information but I would like to stress that this is only one way to address the issues and may not be applicable to everyone's situation, it does however provide a couple of advantages over 'View Extras':
PDFs can be viewed within JRiver MC Standard View and can also be viewed on remote MC library clients.
Disadvantages:
Tagging overhead and Library modification.
To create a new Link, right click an existing Link (such as Amazon) and select Manage links, this will display a dialog screen (see attached screenshot) click Add to create new Link. The fields can then be filled manually or you can use 'Paste Link Information From Clipboard' to use the code provided below.
To view PDF within Standard View ensure 'Action' is set to 'Open inside program'
To address the IsMissing() slow startup problem I decided to create a new Library field which I called 'Booklet' (see attached screenshot -Tools\Options\Library & Folders\Manage Library Fields\Add New Field), this is populated with the filename of the PDF booklet or left blank if no booklet exists. This field can be seen in the Tag window shown in the screenshot of my original post above. It is assumed that the PDF booklet is stored in the same folder as the album files. The 'Booklet' Link will only appear if the 'Booklet' library field is populated (the IsEmpty() expression in the Filter field).
The following Link information will display the PDF within Standard View on a LOCAL instance of MC when the 'Booklet' Link is selected:
<Link version="1.2">
<Item Name="Name">Booklet</Item>
<Item Name="Action">0</Item>
<Item Name="Base URL"/>
<Item Name="Search URL">file:////[Filename (path)][Booklet]</Item>
<Item Name="Filter">[=IsEmpty([Booklet])]=[0]</Item>
</Link>
But what if I view this Library on a client machine? The 'Booklet' Link will appear because the 'Booklet' library field is populated but it will not display anything as the path to PDF file is invalid on the client.
The solution for me was quite simple as my Audio files are all on one disk volume and I have a network share created for the top level folder (so D:\Music is shared as \\MEDIASERVER\Music).
This revised Link information will display PDFs both LOCALLY AND REMOTELY on a Client:
<Link version="1.2">
<Item Name="Name">Booklet</Item>
<Item Name="Action">0</Item>
<Item Name="Base URL"/>
<Item Name="Search URL">file:////Replace([Filename (path)],[Volume Name],MEDIASERVER)[Booklet]</Item>
<Item Name="Filter">[=IsEmpty([Booklet])]=[0]</Item>
</Link>
To use the above code substitute 'MEDIASERVER' with the name of your library server and make sure you have a network share to a top level folder on a volume e.g D:\Music is shared as \\MEDIASERVER\Music.
As I said before, this is what works for me, if you have Music files spread across multiple volumes or below the root level of a volume (such as C:\Users\Yourname\Music) then the Client Link above ain't gonna work.
A solution may be to copy all the PDFs to a separate folder, network share it and modify the Link information accordingly.
The LOCAL 'Booklet' Link will work for most setups but I appreciate populating an additional Library field may be too onerous if you already a have a huge library. I have been using this for years so it is just one extra tagging operation when I import a new album that has a PDF booklet.
I hope this satisfies your interest @iggies.
A final point, it would be good if the MC 'View Extras' could be configured to display PDFs within Standard View (AFAIK it can't at the moment).
Terry