Is it possible to have a PDF file open at a predefined magnification in Adobe Reader?

If you can control the URL used to download, you can put parameters in the URL to control how the built-in reader will display the file.

For example, http://example.org/doc.pdf#zoom=50 will set the magnification to 50%.

See: https://www.adobe.com/content/dam/acom/en/devnet/acrobat/pdfs/pdf_open_parameters.pdf

The above applies to the built-in reader supplied by Adobe. Other readers may not honor the parameters. In particular, see the answer to this question regarding Chrome.

An example of how to define magnification when opening a file (regardless of the default one):

AcroRd32 /A "zoom=50=OpenActions" sh.pdf

First, this is a programming website, so you should identify a programming context. This question will probably be closed because it belongs on the soon to be launched serverfault.com

To set the default magnification, you need Adobe Acrobat Standard or Professional not Reader to have the ability to edit pdfs. Then when you open the document, click File | Properties. Click the Initial View tab and enter 72% in the magnification text box and click ok. Save your pdf and reopen it. It should default to 72% magnification when it is opened.

Note: I am unsure if other open source pdf editors provide this type of functionality.

Update: Standard doesn't work for saving magnifications.