Would be nice if we can draw the box with cursor on a PDF when we want to define the rectangle.
The Acrobat plugin (at least the reader) don't expose this kind of interactivity with the host application.
The only way I see it is to define the view rectangle coordinates as percentages of the page size, or with fixed values if all the documents have the same page size.
But, if you have Acrobat (not the Reader), you can easily create a named destination to zoom in on these drawings legend.
From an Acrobat support article:
"A destination is the end point of a link represented by text in the Destinations tab. Destinations enable you to set navigation paths across a collection of Adobe PDF documents. Linking to a destination is recommended when linking across documents because, unlike a link to a page, a link to a destination is not affected by the addition or deletion of pages within the target document.
To set a Destination in Acrobat 7.0 (Professional or Standard):- Set the Zoom and page location to the way you want it on the screen.
- Select View > Navigation Tabs > Destinations to open the Destinations tab.
- Select Options > New Destination from the Destinations tab.
- Give the Destination a name."
Next time you open the document you just need to click that "Legend" named destination, to show again the same view you used when the destination was created.
Open the PDF in a named destinations is also one of the functions exposed by the Acrobat plugin, so it can be automated by PDFE.