NextCloud

Sync Calendar and Contacts (example iPhone or Mac OS) via CalDAV and CardDAV protocols
Calendar and contacts are added to macOS and iOS in the same way:
First, you need to create an "App Password" for your new connection. Follow this guide to complete this step.
Add Calendar Sync
In the next step, add the calendar to your iPhone / Mac:
Go to Settings -> Calendar -> Accounts -> Add Account -> “Other” -> “Add CalDAV Account”... and fill in the fields as shown below.
Add Contact Sync
To add contact synchronization, go to: Settings -> Calendar -> Accounts -> Add Account -> “Other” -> “Add CardDav Account”... and fill in the fields as shown below.

Create a "App Password"
If you have 2FA (two factor authentification) enabled, there is no way apps like calendar, contacts (trough CalDAV, CardDAV protocols)  or other internet protocols such as WebDAV can sync with your account. To enable this feature, Nextcloud lets you create "App Passwords" trough your account settings. Even if you change your Account's Password, the "App Passwords" so you do not need to re-connect all your clients and apps.
To create a new "App Password", go into your account's security settings:

Connect via WebDAV to your Account
What is WebDAV?
WebDAV allows you to access and manage files on a server as if they were on your local device. You can upload, download, create, move, delete files and folders, and collaborate by managing versions and locking files.
There is a very extensive guide on the topic of WebDAV and Nextcloud which can be found here. 
To connect to your NextCloud via WebDAV, first create an "App Password" (e.g., named "WebDAV") by following this guide. Then, use this URL in your file manager or app:
https://hub.masali.cloud/remote.php/dav/files/YOUR-USER-NAME-HERE/
Do not forget to insert your username you got when creating an "App Password" in place of "YOUR-USER-NAME-HERE"!

View PDFs in the Web Interface
Due to a security setting, the browser times out when clicking on a PDF to open it. A simple workaround is to open the PDF file with Nextcloud Office: