You may be over-thinking this. If the scale is 1:300, all you need to do is multiply the current dimension you want to calculate by 300.
For instance if the storage room is 20mm wide:
2cm × 300 = 600cm = 6m
Your storage room is 6m wide.
If you were only capable of measuring in pixels, you would need to know the PPI of the image to calculate the correct physical measurement, you can however measure physical distances in PDFs relatively easy, that depends on the software you are using though.
In Adobe Acrobat and Adobe Reader, for example, there are measuring tools.
Use the measuring tools to measure distances and areas of objects in PDF documents. The measuring tools are useful for showing the distances and areas associated with objects in a form or computer-aided design (CAD) drawing.
With the scale options of the measuring tools in Acrobat or Reader you won't even have to calculate the measurements, just input the correct scale and the tool will show you the real-life dimensions. Easy.