To reduce the size of documents you may have to change your template and compress image sizes within your document.
If your document has no images and is too large the cause is usually the template that you have used. The simplest solution is to create a new document with no template and cut and paste your text into the new document.
For a Microsoft document, if you have imported a lot images and cropped them to display only the part you want, then the rest of the image is still part of the total document, So to reduce the total filesize, left click on an image, go to the menu bar at the top of the screen, select Picture Tools, Format, Compress Pictures, deselect "Apply only to this picture", and keep select for "Delete cropped areas of pictures" and try "Use document resolution" and then click OK, save the document and check if the document is small enough in size.
A Microsoft document with TIFF or PNG images will not compress, you must first save each of the images as JPG image and then the images can be compressed. Also JPG images are much smaller than TIFF and PNG images
First you need all images in your document saved as JPG images. To do this right click on the image, select save image, if it is not JPG already, then select the File format to be JPG and save the image file.
Next right click on the image, select change image, select the correct image and save back into the document. Save the document to see if it small enough. Repeat for all images in the file and check to see if this is enough to make your document small enough, if not then you will need to do the next step.
Next to compress images stored in a Microsoft document select any JPG image in your file, then go to the menu bar at the top of the screen, Picture Tools, Format, Compress Pictures, deselect "Apply only to this picture", try Print (220ppi) and click OK, save your file and see how large your document is now.
Notes for Mac Word Users
A simple way to make your Word document with pictures smaller is to use your Paintbrush program on your Mac to save all of your images in JPG format, then do NOT use the drag and drop feature as this will change the graphic format, instead use the insert picture option in Word to place these pictures in your document. This alone will considerable reduce the size of your document.
To compress images you can either use Paintbrush on each image, OR use Word, start with all images in JPG format as the images will be small to start with, then select one image, the Format Picture tab should appear, then select on Compress option in the Adjust group, then choose preferred Picture Quality: 220ppi, remove cropped, and Apply to All pictures, then click OK, and save the document.
A second minimal sized PDF document
If you have a large number of scanned PDF or JPG documents that do not need to processed by Turnitin, then another solution is to place all of these scanned documents in a second Word document and then save as a minimal size PDF. In Microsft Word - select File, Export, Create PDF, then select Minimum size (publishing online), and Publish.