John
On the two examples you provided to me at PUG 2018 last week - With the current product if you have no distances to go on and just have the scale - here are some things that you can try
If you open the PDF files in Acrobat Reader and go to File - Properties - you can see the Sheet Size that these are plotted onto (24" x 36") paper size.
When you import the PDF into BC-HCE and you start the placement of the image, you can likely assume that the sheet border (highlighted line of the image) is the 24x36" border and use that to scale the plans if you have nothing else to go on. In this case the Scale says 1" = 20' so a 36" side of the plans is 720' so I use Known Distance and optional bearing if you want to orient the sheet to true north, enter the known distance of 720' and pick the Lower Left and Lower Right corners of the sheet. For the Bearing I would pick the South and North End of the North Arrow and it would rotate the sheet by ~90 degrees. Once the image is placed - I would check a distance e.g. between parking stripes and see what that tells me - in this case it is 9' which is the min spacing for car parking spaces - so looks OK so far.
Note that in C4 there is also a Grid and you may assume that it is a 20' Grid because the scale is 1"=20' but actually if the Sheet Size scaling is correct the Grid is actually a 15' Grid (Go Figure ....)
When I try Sheet C6, there are 3 drawings here all at 1" = 40'. If I use that sheet and scale of the profile Drawing where the Station indicators give me 100', 200', 300', 400' locations - I can use these to scale this drawing - then if I check the Parking Stripes on the Plan Sheet it shows them also to be 9' wide - confirming Sheet C4 scale to be correct.
If I know these sheets go together, then I would likely use C6 first and then place C4 over C6 using points in C6 to Georeference the Plan part showing the Parking lot to the same plan part in C6.
I will check more Sheets from other PDF files, but I believe that if you can read the Sheet Size from Acrobat Reader, you can scale the Sheets using the Sheet Size defined in the PDF file directly (just take the Scale data and the Sheet Size and compute the Long Side of the Sheet to scale - and then check something that you know to be True (like the Parking Bay width) to validate the scale to be correct.
BE AWARE OF HALF SCALE DRAWINGS ,,,,
Hope that this helps - I have asked Product Managers to look at the option to define a Known Scale and look at the Sheet Size in the PDF records to see if we could just put in a scale to scale the plans directly.
This however is a reasonable work around I think.
Alan