Chromium Version 57.0.2987.110 (64-bit): PDFs are not opened with default pdf reader (Open PDF Files In Default PDF Viewer Application is checked) they are just downloaded to disk with no further actions. With "Open PDF Files In Default PDF Viewer Application" unchecked, they are correctly opened within the chrome viewer.

Windows 7 x64

P.S. same behaviour in Chromium 59.x

Re #1: This is currently the intended behavior unless you check the automatically open this file type (from the download bar right of the filename). It used to open them automatically but we considered this to be rather intrusive.

"automatically open this file type" did the job. Thanks for your tip!

I can confirm this is still an issue. Just did the following for a user:
Had issues with English PDF document language displaying in Greek, when viewed with Chrome 54 PDF viewer.
1. User was on Chrome 54
2. Disabled Chrome PDF Viewer via the chrome://plugins page
3. Tested same link to the document and it came up fine in Adobe Reader after the file was saved to local machine
4. Checked same document link on my system, Chrome PDF viewer worked fine, document was in English
5. Checked my version, which was 57
6. Uninstalled Chrome 54 from user's machine
7. Reinstalled new Chrome 57 from Google's website
8. Used document link and it still downloaded the document and it did not come up on Chrome Viewer
9. Went to Content Settings and unchecked "Open PDF Files in the default PDF viewer application"
10. Closed Chrome and reopened it
11. Went to same document link, link STILL downloaded and didn't open in Chrome Viewer
12. Validated Content Settings and option appears to have rechecked itself
13. Unchecked setting again and closed Chrome
14. Open Chrome and looked at the Content Setting again
15. Rechecked itself. WILL NOT UNCHECK!!!

Whatever the previous Chrome version's setting was for the Chrome PDF Viewer, becomes a permanent setting in the newly installed Chrome 57.

Looking into this. I have a suspect about the reason.

https://codereview.chromium.org/2841843002/ Fix landed.

Cr-Commit-Position: refs/heads/master@{#467285}
Committed: https://chromium.googlesource.com/chromium/src/+/e08ac360faf4df6fc67b419d549ad682f8e56962

This is M-60 correct?  Adding that label for now.  Will this be merged back?

Yes, I will check with the PMs if this should be merged back. 
Possibly to M59 but I will take care of putting the right labels for that. Thanks!

Requesting merge to 59. For https://codereview.chromium.org/2841843002/

(I don't know why the commitbot didn't post the landed CL on the bug)

Please tag with applicable OSs.  Thanks!

Your change meets the bar and is auto-approved for M59. Please go ahead and merge the CL to branch 3071 manually. Please contact milestone owner if you have questions.
Owners: amineer@(Android), cmasso@(iOS), gkihumba@(ChromeOS), Abdul Syed@(Desktop)

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/df3bb192e93b863cde928dd9416976180bfb24ae

commit df3bb192e93b863cde928dd9416976180bfb24ae
Author: Julian Pastarmov <pastarmovj@chromium.org>
Date: Fri Apr 28 07:49:27 2017

Remove the enable flag from all plugins after the option has been ported.

https://codereview.chromium.org/2518493002 removed the code to reenable
plugins but this doesn't reset the state and end up overwriting the
new pdf/flash setting each time the old prefs are read again.

After the CL the flag was not added to new Preferences files this code
only applies to existing legacy settings.

BUG= 705979 

Review-Url: https://codereview.chromium.org/2841843002
Cr-Commit-Position: refs/heads/master@{#467285}
(cherry picked from commit e08ac360faf4df6fc67b419d549ad682f8e56962)

Review-Url: https://codereview.chromium.org/2851703002 .
Cr-Commit-Position: refs/branch-heads/3071@{#286}
Cr-Branched-From: a106f0abbf69dad349d4aaf4bcc4f5d376dd2377-refs/heads/master@{#464641}

[modify] https://crrev.com/df3bb192e93b863cde928dd9416976180bfb24ae/chrome/browser/plugins/plugin_prefs.cc