Dockerized Nginx Reverse Proxy to NodeJS App not returning PDFs

I have a dockerized Nginx reverse proxy to a nodejs app which is supposed to serve generated PDFs based on data entered.

Here's the current state of nginx.conf

# nginx.conf

server {
    listen       80;
    server_name  localhost;
    root         /usr/share/nginx/html;
    include      /etc/nginx/mime.types;

    # bot
    location / {
        proxy_pass http://frontend:80;
        proxy_http_version 1.1;
        proxy_set_header X-Forwareded-For $proxy_add_x_forwarded_for;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection "Upgrade";
        proxy_set_header Host $http_host;
        proxy_buffering off;
    }

    location /api {
        proxy_pass http://backend:3000;
        proxy_http_version 1.1;
        proxy_set_header X-Forwareded-For $proxy_add_x_forwarded_for;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection "Upgrade";
        proxy_set_header Host $http_host;
        proxy_buffering off;
    }

    location /socket.io {
        proxy_pass http://backend:3000;
        proxy_http_version 1.1;
        proxy_set_header X-Forwareded-For $proxy_add_x_forwarded_for;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection "Upgrade";
        proxy_set_header Host $http_host;
        proxy_buffering on;
        proxy_buffer_size 8k;
        proxy_buffers 8 8k;
    }

    location /files {
        proxy_pass http://backend:3000;
        proxy_http_version 1.1;
        proxy_set_header X-Forwareded-For $proxy_add_x_forwarded_for;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection "Upgrade";
        proxy_set_header Host $http_host;
        proxy_buffering on;
        proxy_buffer_size 8k;
        proxy_buffers 8 8k;
    }

    # redirect server error pages to the static page /50x.html
    #
    error_page   500 502 503 504  /50x.html;
    location = /50x.html {
        root   /usr/share/nginx/html;
    }
}

In the node app, the correct directory is mapped to the /file endpoint:

app.use('/files', express.static(path.join(__dirname, 'storage')));

However, when trying to load a PDF in browser, I get the following error message:

This site can’t provide a secure connectionlocalhost sent an invalid response.

ERR_SSL_PROTOCOL_ERROR

And the nginx container logs show:

172.23.0.1 - - [24/Oct/2019:16:42:39 +0200] "\x16\x03\x01\x02\x00\x01\x00\x01\xFC\x03\x03\xF7\x01\xBB\x1E\xD2\xAF*\xB7\xFB\xB96b4\xEE\xEB\xC9\x18u\x94-`(\xBA\xB9'\xC18o\xA0n\xA0\xDA \xB2l~n\xDFd\xB9\x02C/]\x9C3\xD5u\x19\xA7\xE3c\x1D\xFD\x06\xEBv\x0B\x88x\x8FW\x9As\x1F\x00\x22\xDA\xDA\x13\x01\x13\x02\x13\x03\xC0+\xC0/\xC0,\xC00\xCC\xA9\xCC\xA8\xC0\x13\xC0\x14\x00\x9C\x00\x9D\x00/\x005\x00" 400 157 "-" "-" "-"

The goal is to be able to test everything on localhost (hence no SSL), production environment would probably include an additional Nginx reverse-proxy in front of this one which handles certifcates etc.

I'd be very grateful for any pointers regarding how to serve PDFs in both environments.

Thanks in advance!