osx - cannot open images or pdfs
Were you using Firefox browser to download them? 

If so in Firefox go to: Tools>Add-ons>Plugins>Adobe Acrobat NPAPi Plugin>Disable.

https://support.mozilla.org/en-US/questions/925055?e=es&amp;as=aaq



If not then right click (control click) on a PDF file. Select "get info". Toward the bottom of the dialog you'll see "Open with:" and an arrow. Click on the arrow and you'll be presented with the applications that can open PDF files. Select Preview and then click on the box that says: "Change All..."

When you select Preview it will say below it: "Use this application to open all documents like this one."