The default in the new Google Drive is to open a PDF in preview mode. If you prefer the way the old Google Drive handled PDF’s there is an app that you may like.
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Google Drive will ask you if you want to use this app as your default for PDF’s. If you choose this then your PDF’s will not show up in the preview mode unless you use the menu option for preview.
PDF viewer has it’s own sharing link. Using this link will allow collaborators to open the document in PDF viewer rather than Google Drive preview. Open the document. In the upper right hand corner is a swoopy arrow to share the PDF.