Problem
For users that have reported issues with the file size when uploading files into platforms such as the Purchase Order System, Blackboard, email, etc. Errors such as "pdf file is too large", using the Adobe Acrobat Pro can be a solution.
Environment
The Adobe Acrobat Pro has an option to optimize a PDF, reducing the size of PDF file. This can be used in any situation where the user needs to have a smaller version of the file that is about to be submitted.
Resolution
In order to solve this issue, there are 3 simple steps that can be done:
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Open a PDF in Acrobat.
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From the hamburger menu 
(Windows) or the File menu (macOS), select Save As Other > Reduced Size PDF.
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Choose the location to save the file and select Save. Acrobat displays a message showing the successful reduction in PDF size.
In case the user wants more information on how to change/manage their files, Adobe oficial website has a page that describe many interesting tools that can be used by Aboble Acrobat Pro users: https://helpx.adobe.com/acrobat/using/optimizing-pdfs-acrobat-pro.html
Cause
The main cause for this error is the file size is too big to be uploaded/sent, so the user needs to diminish it.