For most users, clicking on a link to access a pdf file will open the file immediately in a new browser window or tab. For a smaller number of users, clicking on the link will prompt the user to download the file in order to then open it.
How PDF files open is dependent upon each user's personal browser settings.
Check out the info about browser settings for pdf files in this article:
How to change browser download settings for PDF files.
You can also do a Google search for something like:
'Firefox pdf settings without download'
(replace 'Firefox' with the name of your browser, such as Chrome or Safari).
Here are 2 articles we found: