It seems to be a relatively new feature that pdf files are shown directly in the browser instead of downloading them.
How can I download them instead?
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Dolphin Mini blog post documents how to download the PDF file.
Q2: How to download PDF files?
A2: 1) long press the link of PDF files, choose the “save link” option in context menu. 2) click “download” option at the bottom of the page after you click the link.