The first thing is to set the view to High Quality Display (View > Display Performance > High Quality Display) and then, if you are allowing object level display settings to be sure the image itself is not carrying a different tag (Right click the image and choose dispaly settings). You can set the default dispaly quality in your preferences. High qulaity is much clearer than Typical, the default, but will bog down some older hardware.
If that doesn't solve the problem, you probably have scaled the image in ID beyond the point where it is usable.
Why would saving it as a PDF make things better or worse? A pdf is just a wrapper for files that can contain vector, text, raster, video, interactive elements etc. It's just a container for things - it wouldn't improve the quality of a file just because it's converted to another format.
Images that are Placed into InDesign via File>Place or drag and drop from a folder are only LINKED to the document. It places a PROXY to simulate the layout. You can go to View>Display Performance>High Quality
BUT this might drastically make things run a bit slower - depending on how many images you have. As InDesign is not an image viewer nor is it an image editor.
You can view the QUALITY of the image in PPI (Pixels Per Inch) only - by going to Window Info and selecting the image.
The Info Panel will show you
The Actual PPI refers to the PPI of the image at 100% original size.
Effective PPI shows the PPI the image is as a result of scaling the image - hence this is the OUTPUT PPI of the image - and this is what you need to be concerned with.
If the PPI is too low then you will need to get a higher resolution image.
WEB - 72 ppi
Newspapers - 80-150 ppi
Magazines - 225-300 ppi
High Quality Art/Coffee table stuff - 300-400ppi
The amount of PPI is dependent on the output intent (what printer and what paper etc.)
Check with your print vendor for advice on PPI and what you can actually use.