I found conflicts between Adobe Reader and Preview. PDF forms that I downloaded from websites arrived corrupted and would not open in either Adobe Reader or Preview.
However, this works:
Set Preview as the preferred app to open all PDFs.
To do this, select a PDF, Get Info (cmd i) and towards the bottom (Open with:) choose 'Preview' and click on Change All... to use Preview to open all documents like this one.
On a website, download a PDF (Right Click, Save Linked File As...)
The PDF opens in Safari. Save it to the Desktop or anywhere convenient.
File is saved without corruption. You can open it in Preview or Adobe Reader.
However, if the PDF is a form that you will edit, you must use Adobe Reader. Preview will result in blank fields after you save.
Right Click the new PDF and "Open With" Adobe Reader.
Ignore the message about Adobe not being the preferred app. Click on "No".
I tested a PDF form edited in Adobe Reader by emailing to and from a friend several times. We both edited the entries. All good. Mac and Windows were able to communicate.
So, the answer for me is to leave Preview as the preferred app for all PDFs.
Regards,
Ian.