Update

Both Edge for Android and Firefox for Android have native support for opening PDF files directly in the browser.

Original answer

You can use a Google Docs page to view PDF files in the browser. To do this, visit the page https://docs.google.com/viewerng/viewer?url=http://yourfile.pdf, where http://yourfile.pdf is the path to the PDF file you want to view (credits to this answer).

Unfortunately, there is as far as I know no way to do this automatically in Chrome (you can always paste the URL manually every time you want to open a PDF file but that's not very practical). However, if you use a browser that supports extensions, it's possible to use an extension to automatically redirect PDF files to this URL. Unfortunately, you can't do this on any well-known browsers (Chrome and Edge for Android don't support extensions at all, and Firefox for Android only supports a very limited set of extensions), but there are less well-known browsers where you can do this. The one I use is Kiwi (I have no association with its author).

Once you've installed Kiwi (or another browser that supports Chrome extensions), install the Redirector extension (I have no association with its author). Next, click on the three dots on the top right, scroll down, click "Redirector", then click on the "Edit Redirects" button, and then click on "Create new redirect":

That will open a popup with five text boxes and two checkboxes. Enter the following information:

1. The first box is just a description, you can enter whatever you want, for example Open PDF in browser.
2. The second box is an example URL to a PDF file, you can enter the URL to whatever PDF file you want.
3. In the third box, enter (^https?://[^?]+\.pdf($|\?).*$). This is a regular expression that matches URLs of PDF files. If you know regular expressions, you may want to note that the parentheses that enclose the whole regular expression are necessary to capture the whole URL so that the $1 in the next box works as expected.
4. In the fourth box, enter https://docs.google.com/viewerng/viewer?url=$1. This is the URL that PDF files will be redirected to, $1 represents the URL of the original PDF file.
5. Check the box labelled "Regular Expression".
6. The last box is just another description Again, enter whatever you want, for example, Redirects PDF files to a Google docs page that shows the PDF in the browser.

When you're done entering all that, it should look like this:

Now click Save and you're done. Now all PDF files will be opened in the browser like this:

To get a link to the original PDF file, copy the URL from the title bar and remove the https://docs.google.com/viewerng/viewer?url= part from the beginning.

I think that the problem sometimes occurs when the web server you are downloading the PDF from does not serve the PDF with the correct MIME type.

When the web server serves a PDF with the correct MIME type, Chrome previews it. When the web server serves a PDF with the incorrect or generic MIME type (application/octet-stream), Chrome downloads it.

If the server returns a Content-Type of text/html Chrome will display/open the PDF directly in the browser. However, if the server returns a Content-Type of application/pdf then the browser will prompt the user to save/download the PDF.

You can open a file PDF in Google Docs Viewer by appending the URL to:

https://docs.google.com/gview?embedded=true&url=<URL of a supported doc>

This would open a PDF file in the default browser or a WebView.

A list of supported formats is given here.

Is anyone else getting completely fed up with these companies just screwing regular workers by doing the dumbest things you can imagine? For example, not being able to open a .PDF in the default application from a web browser. Like users have been able to do for years? The options are open in the browser or download. WTF is this? At first, I thought it was just Edge, nope it’s Chrome as well now. This keeps coming up more and more and I can’t seem to find a simple solution.

I have a client that is literally about to explode. They don’t want to have to download the pdf and then open it. They haven’t had to do it in years and they don’t want to do it now. Nor are they interested in using edge or any other browser to deal with PDF’s. They honestly open like 50 to 60 pdf’s a day and need all of the stuff in Adobe Pro. It’s literally what they do all day. Now it’s an extra step for god only knows why.

Now it’s on me to find a workaround because these dumbass companies (mainly Microsoft) just don’t care about their customers. Sorry I’m bitching but it’s gotten to a completely unacceptable level of complete ridiculousness.

Does anyone know why they are doing this? IE, a security reason. I’m trying to find a reason to explain to clients why it’s being done. Adobe is a $285 Billion company for a reason. People use the crap out of their software. That probably answers my question of why this happening, money. The clients don’t care about the tech. They just want to get some work done the same way they did it before. Not with extra hoops and jacking around.

So frustrated.